Lithium-ion storage battery and electronic device

ABSTRACT

To provide a storage battery including a carbon-based material. To provide a graphene compound film having desired ion conductivity and mechanical strength while preventing direct contact between electrodes in a storage battery. To achieve long-term reliability. A lithium-ion storage battery includes a positive electrode, a negative electrode, an exterior body, and a separator between the positive electrode and the negative electrode. In the lithium-ion storage battery, one of the positive electrode and the negative electrode is wrapped in a first film, and the positive electrode, the negative electrode, and the separator are stored in the exterior body. The first film may include a first region in which the first film includes a first functional group. The first film may further include a second region in which the first film includes a second functional group different from the first functional group. The first film may be a graphene compound film.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a lithium-ion storagebattery and an electronic device.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, one embodimentof the present invention relates to a process, a machine, manufacture,or a composition of matter. Specifically, examples of the technicalfield of one embodiment of the present invention disclosed in thisspecification include a semiconductor device, a display device, a liquidcrystal display device, a light-emitting device, a lighting device, apower storage device, a storage device, a method for driving any ofthem, and a method for manufacturing any of them.

2. Description of the Related Art

In recent years, a variety of power storage devices, for example,storage batteries such as lithium-ion storage batteries, lithium-ioncapacitors, and air cells have been actively developed. In particular,demand for lithium-ion storage batteries with a high output and a highenergy density has rapidly grown with the development of thesemiconductor industry, for electronic devices, for example, portableinformation terminals such as mobile phones, smartphones, and laptopcomputers, portable music players, and digital cameras; medicalequipment; next-generation clean energy vehicles such as hybrid electricvehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electricvehicles (PHEVs); and the like. The lithium-ion storage batteries areessential as rechargeable energy supply sources for today's informationsociety.

The performance required for the lithium-ion storage batteries includesincreased energy density, improved cycle life, safe operation under avariety of environments, and longer-term reliability.

Furthermore, flexible display devices used while being mounted on humanbodies or curved surfaces, such as display devices mounted on heads(head-mounted displays), have been proposed in recent years. This hasincreased demands for flexible storage batteries that can be mounted oncurved surfaces to be used together with flexible display devices.

An example of the lithium-ion storage battery includes at least apositive electrode, a negative electrode, and an electrolyte solution(Patent Document 1).

Owing to excellent electric characteristics such as high conductivity orhigh mobility and excellent physical characteristics such as sufficientflexibility and high mechanical strength, application of graphene to avariety of products has been attempted recently (see Patent Documents 2to 4).

Here, in a commercially available storage battery, which is arechargeable power storage device, a carbon-based material such asgraphite is used for a negative electrode. Graphite has a crystalstructure where sheets of carbons which have sp² hybrid orbitals and areregularly arranged two-dimensionally are stacked. The storage battery ischarged and discharged utilizing occlusion of lithium ions from apositive electrode into a gap between sheets of carbons in the crystalstructure.

Carbon-based materials are advantageous in achieving lighter storagebatteries and are highly safe as materials, suggesting the necessity ofthe wider application of carbon-based materials to storage batteries.

REFERENCES Patent Documents

-   [Patent Document 1] Japanese Published Patent Application No.    2012-009418-   [Patent Document 2] United States Published Patent Application No.    2011/0070146-   [Patent Document 3] United States Published Patent Application No.    2009/0110627-   [Patent Document 4] United States Published Patent Application No.    2007/0131915

SUMMARY OF THE INVENTION

A separator is provided between a positive electrode and a negativeelectrode and has a function of separating the electrodes. If theelectrodes in the lithium-ion storage battery are short-circuited, anuncontrollable high current flows between the electrodes, and, forexample, a large amount of heat is generated, causing a safety hazard insome cases. Even when a safety hazard is not caused, self-dischargeoccurs to cause deterioration and a function as a battery is impaired.

Furthermore, in a process of manufacturing or charging/discharging thelithium-ion storage battery, some of carrier ions contributing tocharging/discharging are deposited on a surface of the negativeelectrode and becomes an irreversible component, which impairs afunction as a battery. When the deposition of lithium on the surface ofthe negative electrode proceeds greatly, the deposited lithium becomes awhisker-like structure (whisker) and grows in some cases. The structuremight pass through a pore in the separator to cause a short-circuitbetween the electrodes depending on the property of the separator, whichalso causes a problem.

Furthermore, in a flexible lithium-ion storage battery, various kinds ofstress are generated inside the storage battery in accordance withchange in the shape of the storage battery. In the case where thestorage battery does not have a structure for relieving the stress,shear failure occurs easily at a portion of the storage battery, so thata function as a storage battery is lost.

A carbon-based material potentially has excellent properties as amaterial. When such a carbon-based material is used, a lightweight andsafe storage battery with high quality can be provided.

In view of the above, an object of one embodiment of the presentinvention is to provide a lithium-ion storage battery including acarbon-based material. Another object of one embodiment of the presentinvention is to provide a storage battery including a graphene compoundfilm having desired ionic conductivity and mechanical strength whilepreventing direct contact between electrodes in the storage battery.Another object of one embodiment of the present invention is to achievelong-term reliability of a storage battery.

Another object of one embodiment of the present invention is to providea storage battery including a novel graphene compound film. Anotherobject of one embodiment of the present invention is to provide a novelpower storage device or the like.

Another object of one embodiment of the present invention is to providea storage battery that can change in shape, i.e., a storage batteryhaving flexibility. Another object of one embodiment of the presentinvention is to provide a novel storage battery having flexibility andincluding a novel graphene compound film that can resist change inshape.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

A structure of one embodiment of the invention disclosed in thisspecification is a lithium-ion storage battery including a positiveelectrode, a negative electrode, and an exterior body. At least one ofthe positive electrode and the negative electrode is at least partlywrapped in a first film. The first film includes a graphene compound.The positive electrode and the negative electrode are stored in theexterior body.

In one embodiment of the invention disclosed in this specification, agraphene compound is a compound where graphene or multilayer graphene ismodified with an atom other than carbon or an atomic group with an atomother than carbon. A graphene compound may be a compound where grapheneor multilayer graphene is modified with an atomic group composed mainlyof carbon, such as an alkyl group or alkylene. For example, graphene andoxygen may be included in a graphene compound, or graphene oxide may beused.

A structure of another embodiment of the invention disclosed in thisspecification is a lithium-ion storage battery including a positiveelectrode, a negative electrode, and an exterior body. A separator islocated between the positive electrode and the negative electrode. Atleast one of the positive electrode and the negative electrode is atleast partly wrapped in a first film. The first film includes a graphenecompound. The positive electrode, the negative electrode, and theseparator are stored in the exterior body.

Note that in one embodiment of the present invention, the first film ofthe lithium-ion storage battery may include a first region in which thefirst film includes a first functional group. The first film of thelithium-ion storage battery may further include a second region in whichthe first film includes a second functional group different from thefirst functional group.

In one embodiment of the present invention, the first film of thelithium-ion storage battery may include a first region in which thefirst film is subjected to first modification. The first film of thelithium-ion storage battery may further include a second region in whichthe first film is subjected to second modification different from thefirst modification. Note that in the lithium-ion storage battery, thefirst film may be a graphene oxide film.

A structure of another embodiment of the invention disclosed in thisspecification is a lithium-ion storage battery including a positiveelectrode, a negative electrode, and an exterior body. The positiveelectrode is at least partly wrapped in a first film. The negativeelectrode is at least partly wrapped in a second film. The first filmincludes a graphene compound. The second film includes a graphenecompound. The positive electrode and the negative electrode are storedin the exterior body.

A structure of another embodiment of the invention disclosed in thisspecification is a lithium-ion storage battery including a positiveelectrode, a negative electrode, and an exterior body. A separator islocated between the positive electrode and the negative electrode. Thepositive electrode is at least partly wrapped in a first film. Thenegative electrode is at least partly wrapped in a second film. Thefirst film includes a graphene compound. The second film includes agraphene compound. The positive electrode, the negative electrode, andthe separator are stored in the exterior body.

In one embodiment of the present invention, the first film of thelithium-ion storage battery may include a first region in which thefirst film includes a first functional group. The first film of thelithium-ion storage battery may further include a second region in whichthe first film includes a second functional group different from thefirst functional group. The second film of the lithium-ion storagebattery may include a third region in which the second film includes athird functional group. The second film of the lithium-ion storagebattery may include a fourth region in which the second film includes afourth functional group different from the third functional group.

In one embodiment of the present invention, the first film of thelithium-ion storage battery may include a first region in which thefirst film is subjected to first modification. The first film of thelithium-ion storage battery may further include a second region in whichthe first film is subjected to second modification different from thefirst modification. The second film of the lithium-ion storage batterymay further include a third region in which the second film is subjectedto third modification. The second film of the lithium-ion storagebattery may further include a fourth region in which the second film issubjected to fourth modification different from the third modification.Note that in the lithium-ion storage battery, the first film may be agraphene oxide film and the second film may be a graphene oxide film.

In one embodiment of the present invention, the lithium-ion storagebattery may have flexibility.

One embodiment of the present invention can provide a lithium-ionstorage battery including a carbon-based material. One embodiment of thepresent invention can provide a storage battery including a graphenecompound film having desired ionic conductivity and mechanical strengthwhile preventing direct contact between electrodes in the storagebattery. One embodiment of the present invention can achieve long-termreliability of a storage battery.

One embodiment of the present invention can provide a lithium-ionstorage battery including a novel graphene compound film. One embodimentof the present invention can provide a novel power storage device or thelike.

One embodiment of the present invention can provide a storage batterythat can change in shape, i.e., a storage battery having flexibility.One embodiment of the present invention can provide a novel graphenecompound film that can resist change in shape in a storage batteryhaving flexibility.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate lithium-ion storage batteries.

FIGS. 2A and 2B are schematic cross-sectional views of lithium-ionstorage batteries.

FIGS. 3A and 3B are schematic cross-sectional views of lithium-ionstorage batteries.

FIGS. 4A to 4C are schematic cross-sectional views of inner structuresof lithium-ion storage batteries.

FIGS. 5A to 5D show a negative electrode wrapped in a graphene oxidefilm.

FIGS. 6A and 6B illustrate the assembly of a lithium-ion storagebattery.

FIGS. 7A to 7D illustrate the radius of curvature.

FIGS. 8A to 8C illustrate the radius of curvature.

FIGS. 9A to 9C illustrate a coin-type storage battery.

FIGS. 10A and 10B illustrate a cylindrical storage battery.

FIGS. 11A and 11B illustrate a laminated storage battery.

FIG. 12 is an external view of a storage battery.

FIG. 13 is an external view of a storage battery.

FIGS. 14A to 14C illustrate a method for fabricating a storage battery.

FIGS. 15A to 15E illustrate flexible storage batteries.

FIGS. 16A and 16B illustrate an example of a storage battery.

FIGS. 17A1, 17A2, 17B1, and 17B2 illustrate examples of a storagebattery.

FIGS. 18A and 18B each illustrate an example of a storage battery.

FIGS. 19A and 19B each illustrate an example of a storage battery.

FIG. 20 illustrates an example of a storage battery.

FIGS. 21A and 21B each illustrate an application mode of a storagebattery.

FIG. 22 is a block diagram illustrating one embodiment of the presentinvention.

FIGS. 23A to 23C are each a conceptual diagram illustrating oneembodiment of the present invention.

FIG. 24 is a circuit diagram illustrating one embodiment of the presentinvention.

FIG. 25 is a circuit diagram illustrating one embodiment of the presentinvention.

FIGS. 26A to 26C are each a conceptual diagram illustrating oneembodiment of the present invention.

FIG. 27 is a block diagram illustrating one embodiment of the presentinvention.

FIG. 28 is a flow chart showing one embodiment of the present invention.

FIGS. 29A and 29B are schematic cross-sectional views of lithium-ionstorage batteries.

FIGS. 30A to 30C are schematic cross-sectional views of electrodes andgraphene oxide films.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below in detailwith reference to the drawings. However, the present invention is notlimited to the description below, and it is easily understood by thoseskilled in the art that modes and details disclosed herein can bemodified in various ways. Further, the present invention is notconstrued as being limited to description of the embodiments.

Note that in each drawing described in this specification, the size, thethickness, or the like of each component such as a positive electrode, anegative electrode, an active material layer, a separator, an exteriorbody, and the like is exaggerated for clarity in some cases. Therefore,the sizes of the components are not limited to the sizes in the drawingsand relative sizes between the components.

In this specification and the like, ordinal numbers such as “first”,“second”, and “third” are used for convenience and do not denote theorder of steps or the stacking order of layers. Therefore, for example,description can be made even when “first” is replaced with “second” or“third”, as appropriate. In addition, the ordinal numbers in thisspecification and the like are not necessarily the same as those whichspecify one embodiment of the present invention.

Note that in the structures of the present invention described in thisspecification and the like, the same portions or portions having similarfunctions in different drawings are denoted by the same referencenumerals, and description of such portions is not repeated. Further, thesame hatching pattern is applied to portions having similar functions,and the portions are not especially denoted by reference numerals insome cases.

In this specification, flexibility refers to a property of an objectbeing flexible and bendable. In other words, it is a property of anobject that can be deformed in response to an external force applied tothe object, and elasticity or restorability to the former shape is nottaken into consideration. A flexible storage battery can be changed inform in response to an external force. A flexible storage battery can beused with its shape fixed in a state of being changed in form, can beused while repeatedly changed in form, and can be used in a state of notchanged in form. In this specification and the like, the inside of anexterior body refers to a region surrounded by the exterior body of alithium-ion storage battery, in which a structure such as a positiveelectrode, a negative electrode, an active material layer, and aseparator, and an electrolytic solution are stored.

In this specification, modification refers to changing of a function ora property of a graphene oxide film by chemically changing the grapheneoxide film. It may refer to addition of a functional group having acertain function or property.

Contents described in Detailed Description of the Invention can becombined with each other as appropriate.

Embodiment 1

In this embodiment, a lithium-ion storage battery 100 of one embodimentof the present invention and a method for fabricating the same will bedescribed. The case of using graphene oxide is described as oneembodiment of the present invention. The graphene oxide is an example ofa graphene compound.

FIG. 1A shows the lithium-ion storage battery 100 of one embodiment ofthe present invention. The lithium-ion storage battery 100 includes apositive electrode 101, a negative electrode 102, a graphene oxide film103, and a separator 109 that are stored in an exterior body 107. Notethat in the case where the graphene oxide film 103 has a function of aseparator, the separator 109 can be omitted. FIG. 1B shows the casewhere the separator 109 is omitted. The positive electrode 101 iselectrically connected to a positive electrode lead 104 and the negativeelectrode 102 is electrically connected to a negative electrode lead105.

FIG. 2A is a cross-sectional view of the lithium-ion storage battery 100of one embodiment of the present invention along the line A1-A2 in FIG.1A and an enlarged view thereof. The lithium-ion storage battery 100described in this embodiment includes an electrolyte solution 106, thepositive electrode 101, the negative electrode 102, the graphene oxidefilm 103, and the separator 109 as shown in FIG. 2A. Note that thenumber of positive electrodes, that of negative electrodes, that ofgraphene oxide films, and that of separators in the lithium-ion storagebattery 100 described in this embodiment are each mainly 1, but oneembodiment of the present invention is not limited thereto. The positiveelectrode 101 includes a positive electrode current collector 101 a anda positive electrode active material layer 101 b, and the negativeelectrode 102 includes a negative electrode current collector 102 a anda negative electrode active material layer 102 b. FIG. 29A is across-sectional view taken along the line B1-B2 in FIG. 1A. FIG. 29B isa cross-sectional view taken along the line B1-B2 in FIG. 1B.

In the lithium-ion storage battery 100 described in this embodiment, thenegative electrode 102 is wrapped in the graphene oxide film 103 asshown in FIG. 2A. However, one embodiment of the present invention isnot limited thereto, and the positive electrode 101 may be wrapped inthe graphene oxide film 103. Furthermore, each of the positive electrode101 and the negative electrode 102 may be wrapped in a graphene oxidefilm. In the lithium-ion storage battery 100 shown in FIG. 3A, each ofthe positive electrode 101 and the negative electrode 102 is wrapped ina graphene oxide film.

FIG. 2B shows the case where the lithium-ion storage battery 100 of oneembodiment of the present invention does not include a separator. Across section of the storage battery along the line A1-A2 in FIG. 1B andan enlarged view of the cross section are shown in FIG. 2B. In thelithium-ion storage battery 100 described in this embodiment and shownin FIG. 2B, the negative electrode 102 is wrapped in the graphene oxidefilm 103 as in FIG. 2A. However, one embodiment of the present inventionis not limited thereto, and the positive electrode 101 may be wrapped inthe graphene oxide film 103. Furthermore, each of the positive electrode101 and the negative electrode 102 may be wrapped in a graphene oxidefilm. In the lithium-ion storage battery 100 shown in FIG. 3B, each ofthe positive electrode 101 and the negative electrode 102 is wrapped ina graphene oxide film.

In one embodiment of the present invention, the graphene oxide film 103can have a flat surface with a low coefficient of friction. In thiscase, even when the lithium-ion storage battery 100 is deformed, thecomponents in the lithium-ion storage battery 100 can slide on eachother; therefore, damage due to stress is less likely to occur, and thedurability of the storage battery is increased. Moreover, when thecomponents slide on each other, one or each of the positive electrode101 and the negative electrode 102 is wrapped in the graphene oxide filmand is thus not exposed. Therefore, the short-circuit between theelectrodes can be avoided, which can increase the safety of thelithium-ion storage battery 100.

In the lithium-ion storage battery 100 with flexibility and having alaminated structure of one embodiment of the present invention, when thelithium-ion storage battery 100 is deformed, the exterior body and aninner structure (a structure inside the exterior body) are alsodeformed, and stress due to the deformation is applied. A state beforeand after the deformation of the inner structure of the lithium-ionstorage battery having a laminated structure is described with referenceto FIGS. 4A to 4C.

FIG. 4A shows a cross section of the inner structure of the lithium-ionstorage battery 100 including the positive electrode 101 wrapped in thegraphene oxide film and the negative electrode 102 wrapped in thegraphene oxide film. The separator 109 is provided between the positiveelectrode 101 and the negative electrode 102. FIG. 4B is across-sectional view showing a state of an inner structure when thelithium-ion storage battery with flexibility and having a laminatedstructure is deformed. In FIG. 4B, each of the positive electrode 101and the negative electrode 102 is wrapped in the graphene oxide film103, and the surface of the graphene oxide film 103 is smoother and hasa lower coefficient of friction than a surface of the electrode overwhich an active material is formed. Therefore, the positive electrode101, the negative electrode 102, and the separator 109 can slide on eachother easily. Accordingly, stress to be caused by the deformation of thestorage battery is relieved between the positive electrode 101, thenegative electrode 102, and the separator 109, so that damage due to thestress is less likely to occur. Note that damage to the graphene oxidefilm 103 due to the stress is less likely to occur because the grapheneoxide film 103 itself has elasticity. Furthermore, the positiveelectrode 101 and the negative electrode 102 can be prevented from beingexposed to the electrolyte solution and short-circuited because each ofthe positive electrode 101 and the negative electrode 102 is wrapped inthe graphene oxide film 103.

Thus, the use of the graphene oxide film 103 for wrapping each of thepositive electrode 101 and the negative electrode 102 can achieve thelithium-ion storage battery having flexibility and high durabilityagainst bending.

Next, a method for fabricating the storage battery of one embodiment ofthe present invention will be described. Note that in the descriptionbelow, the lithium-ion storage battery shown in FIG. 2A is mainlydescribed unless otherwise specified. However, needless to say, thedescription in this embodiment can be referred to for lithium-ionstorage batteries shown in other drawings.

[1. Graphene Oxide Film]

The graphene oxide film 103 may be formed by oxidizing a graphenecompound or may be a modified graphene oxide. Alternatively, thegraphene oxide film 103 may be obtained by forming a sheet-like grapheneoxide and holding an electrode in the sheet-like graphene oxide. FIGS.5A to 5D show the process. The negative electrode 102 in which anegative electrode active material layer is formed on a negativeelectrode current collector and to which the lead is attached isprepared in advance. A sheet-like graphene oxide is folded, and in astate where the negative electrode 102 is wrapped in the foldedsheet-like graphene oxide, bonding is performed by providing a bondingportion 108 in an outer edge portion of the sheet-like graphene oxide.Thus, the graphene oxide film 103 wrapping the negative electrode 102can be formed. In this case, at least part of the negative electrodeneeds to be prevented from being wholly covered with the graphene oxidefilm 103 to be connected to the lead. Therefore, needless to say, thelithium-ion storage battery of one embodiment of the present inventionis not limited to the lithium-ion storage battery in which the negativeelectrode 102 is wholly wrapped in the graphene oxide film 103. It issufficient that at least part of the negative electrode 102 is wrapped.In addition, the lithium-ion storage battery of one embodiment of thepresent invention is not limited to the lithium-ion storage battery inwhich the graphene oxide film 103 and the negative electrode 102 aredirectly in contact with each other, and another structure may beprovided between the graphene oxide film 103 and the negative electrode102. Note that when the negative electrode 102 is wrapped in thesheet-like graphene oxide film 103, the bonding portion 108 need not beprovided. Examples of a schematic cross-sectional view of the negativeelectrode wrapped in the graphene oxide film 103 are shown in FIGS. 30Ato 30C.

As another method for forming the graphene oxide film 103, the grapheneoxide film 103 may be formed in the following manner: an electrode issoaked in and taken out from a liquid disperse medium in which particlesof graphene oxide are dispersed, and the disperse medium is removed froma surface of the electrode. Alternatively, a cast method may be used.That is, a liquid disperse medium in which particles of graphene oxideare dispersed is applied to an electrode and spread on a surface of theelectrode, and the disperse medium is removed, whereby the grapheneoxide film 103 can be formed.

The negative electrode is wrapped in the sheet-like graphene oxide inthe above example. In the case of the positive electrode, the positiveelectrode is wrapped as in the example. In one embodiment of the presentinvention, one or each of the positive electrode and the negativeelectrode is wrapped in the graphene oxide film 103. In some cases,graphene that is not oxidized can be used as the graphene oxide film103. A graphene compound that can be used as the graphene oxide film 103will be described in detail below.

With repetition of charge of a storage battery including lithium,lithium is deposited on a negative electrode in some cases. Particularlywhen the deposited lithium has a needle-like shape, a short-circuit islikely to occur between the negative electrode and the positiveelectrode through the deposited lithium. Meanwhile, the graphene oxidefilm 103 has a flat surface with a low coefficient of friction. When thenegative electrode 102 is covered with the graphene oxide film 103having the flat surface with the low coefficient of friction, a surfaceof the negative electrode active material layer 102 b and the grapheneoxide film 103 slide on each other in bending and stretching operationof the lithium-ion storage battery 100, so that lithium deposited on thesurface of the negative electrode active material layer 102 b can beremoved physically. This can prevent a short-circuit between thepositive electrode 101 and the negative electrode 102 and accordinglyprevent a function of the lithium-ion storage battery 100 from beingdegraded. Moreover, the reliability of the lithium-ion storage battery100 can be improved. Particularly in the case where the negativeelectrode active material layer 102 b is provided on both surfaces ofthe negative electrode current collector 102 a, lithium deposited on thesurfaces of the negative electrode active material layer 102 b can beremoved at the same time by bending and stretching the lithium-ionstorage battery 100. When the lithium-ion storage battery 100 is bentand stretched intentionally, the above-described effect can be furtherincreased.

The graphene oxide film 103 has the flat surface with the lowcoefficient of friction and therefore can easily slide also in a portionwhere the graphene oxide film 103 contacts with a structure other thanthe negative electrode active material layer 102 b. Therefore, whenstress is generated in the structures by the deformation of the storagebattery, the structures can easily slide on each other in accordancewith the stress, so that the stress is likely to be relieved.Accordingly, the storage battery has high strength against thedeformation.

Note that the case where the negative electrode 102 is covered with thegraphene oxide film 103 is described here; however, one embodiment ofthe present invention is not limited thereto. For example, the negativeelectrode 102 need not be covered with the graphene oxide film 103; thepositive electrode 101 may be covered with the graphene oxide film 103instead of the negative electrode 102. Alternatively, as shown in FIGS.3A and 3B, the positive electrode 101 may also be covered with thegraphene oxide film 103 in a manner similar to that of the negativeelectrode 102.

At least part of the graphene oxide film 103 may be modified. Astructure in which only a certain part of the graphene oxide film 103 ismodified and the other part thereof is not modified may be used. Astructure in which part and the other part of the graphene oxide film103 are modified differently may be used. For example, it is desirablethat a portion of the graphene oxide film 103 between the positiveelectrode and the negative electrode can transmit lithium ions easilyand the other portion can reliably prevent a short-circuit between theelectrodes. Thus, in some cases, it is desirable that modificationstates of a first region and a second region in the graphene oxide film103 be different from each other.

Note that in this specification, the expression “modification state”refers to a state of modification for a graphene compound. Theexpression “two regions are in different modification states” refers tonot only the case where the types of modification made on the tworegions are different from each other but also the case where the sametype of modification is made on the two regions and the strengths of themodification are different from each other. Also in the case wheremodification is made on one region and modification is not made onanother region, the expression “the regions are in differentmodification states” is used. Thus, in some cases, two regions indifferent modification states differ in the kind of an atom or an atomicgroup introduced into a graphene compound, and even in the case whereatoms or atomic groups of the same kind are introduced, the introductionamounts are different from each other.

FIG. 4C shows the case where the modification states of a first regionand a second region of the graphene oxide film 103 are different fromeach other. In FIG. 4C, for example, a first region 103 a can be in amodification state where lithium ions are transmitted easily, and asecond region 103 b can be in a modification state where the mechanicalstrength is high.

Note that the modification of a graphene compound including grapheneoxide will be described in detail below.

[2. Graphene Compound]

In one embodiment of the present invention, one or each of the positiveelectrode and the negative electrode is wrapped in the graphene oxidefilm 103. However, the graphene oxide film 103 is not limited tographene oxide, and other graphene compounds can be used in some cases.A graphene compound may be used for a structure other than the grapheneoxide film 103. For example, a graphene compound can be included in atleast one of the positive electrode current collector 101 a, thepositive electrode active material layer 101 b, the negative electrodecurrent collector 102 a, the negative electrode active material layer102 b, the separator 109, the exterior body 107, and the electrolytesolution 106. As described later, when modification is performed, thestructure and characteristics of a graphene compound can be selectedfrom a wider range of alternatives. Thus, a preferable property can beexhibited in accordance with a component in which a graphene compound isto be used. Moreover, a graphene compound has high mechanical strengthand therefore can be used in a component of a flexible power storagedevice. Graphene compounds are described below.

Graphene has carbon atoms arranged in one atomic layer. A π bond existsbetween the carbon atoms. Graphene including two or more and hundred orless layers is referred to as multilayer graphene in some cases. Thelength in the longitudinal direction or the length of the major axis ina plane in each of graphene and multilayer graphene is greater than orequal to 50 nm and less than or equal to 100 μm or greater than or equalto 800 nm and less than or equal to 50 μm.

In this specification and the like, a compound including graphene ormultilayer graphene as a basic skeleton is referred to as a graphenecompound. Graphene compounds include graphene and multilayer graphene.

Graphene compounds are detailed below.

A graphene compound is, for example, a compound where graphene ormultilayer graphene is modified with an atom other than carbon or anatomic group with an atom other than carbon. A graphene compound may bea compound where graphene or multilayer graphene is modified with anatomic group composed mainly of carbon, such as an alkyl group oralkylene. An atomic group that modifies graphene or multilayer grapheneis referred to as a substituent, a functional group, a characteristicgroup, or the like in some cases. Modification in this specification andthe like refers to introduction of an atom other than carbon, an atomicgroup with an atom other than carbon, or an atomic group composed mainlyof carbon to graphene, multilayer graphene, a graphene compound, orgraphene oxide (described later) by a substitution reaction, an additionreaction, or other reactions.

Note that the surface side and the rear surface side of graphene may bemodified with different atoms or atomic groups. In multilayer graphene,multiple layers may be modified with different atoms or atomic groups.

An example of the above-described graphene modified with an atom or anatomic group is graphene or multilayer graphene that is modified withoxygen or a functional group containing oxygen. Examples of a functionalgroup containing oxygen include an epoxy group, a carbonyl group such asa carboxyl group, and a hydroxyl group. A graphene compound modifiedwith oxygen or a functional group containing oxygen is referred to asgraphene oxide in some cases. In this specification, graphene oxidesinclude multilayer graphene oxides.

As an example of modification of graphene oxide, silylation of grapheneoxide is described. First, in a nitrogen atmosphere, graphene oxide isput in a container, n-butylamine (C₄H₉NH₂) is added to the container,and stirring is performed for one hour with the temperature kept at 60°C. Then, toluene is added to the container, alkyltrichlorosilane isadded thereto as a silylating agent, and stirring is performed in anitrogen atmosphere for five hours with the temperature kept at 60° C.Then, toluene is further added to the container, and the resultingsolution is suction-filtrated to give a solid powder. The powder isdispersed in ethanol, and the resulting solution is suction-filtered togive a solid powder. The powder is dispersed in acetone, and theresulting solution is suction-filtered to give a solid powder. A liquidof the solid powder is vaporized to give silylated graphene oxide.

Note that silylation is described as an example of the modificationperformed on graphene oxide, but silylation is not limited to themodification performed on graphene oxide. In some cases, silylation canbe performed on graphene that is not oxidized. Furthermore, modificationin this embodiment is not limited to the modification performed ongraphene oxide and can be performed on graphene compounds in some cases.The modification is not limited to silylation, and silylation is notlimited to the above-described method.

The modification is not limited to introduction of an atom or an atomicgroup of one kind, and the modification of two or more types may beperformed to introduce atoms or atomic groups of two or more kinds. Asmodification, a reaction of adding hydrogen, a halogen atom, ahydrocarbon group, an aromatic series hydrocarbon group, and/or aheterocyclic compound group may be performed. As a reaction ofintroducing an atomic group to graphene, an addition reaction, asubstitution reaction, or the like are given. Alternatively, aFriedel-Crafts reaction, a Bingel reaction, or the like may beperformed. A radical addition reaction may be performed on graphene, anda ring may be formed between graphene and an atomic group by acycloaddition reaction.

By introducing a given atomic group to a graphene compound, the physicalproperty of the graphene compound can be changed. Therefore, byperforming desirable modification in accordance with the application ofa graphene compound, a desired property of the graphene compound can beexhibited intentionally.

A formation method example of graphene oxide is described below.Graphene oxide can be obtained by oxidizing the aforementioned grapheneor multilayer graphene. Alternatively, graphene oxide can be obtained bybeing separated from graphite oxide. Graphite oxide can be obtained byoxidizing graphite. The graphene oxide may be further modified with theabove-mentioned atom or atomic group.

A compound that can be obtained by reducing graphene oxide is referredto as reduced graphene oxide (RGO) in some cases. In RGO, in some cases,all oxygen atoms contained in the graphene oxide are not extracted andpart of them remains in a state of bonded oxygen or atomic groupcontaining oxygen. In some cases, RGO includes a functional group, e.g.,an epoxy group, a carbonyl group such as a carboxyl group, or a hydroxylgroup.

A graphene compound may have a sheet-like shape where a plurality ofgraphene compounds overlap each other. Such a graphene compound isreferred to as graphene compound sheet in some cases. The graphenecompound sheet has, for example, an area with a thickness larger than orequal to 0.33 nm and smaller than or equal to 10 mm, preferably largerthan or equal to 0.34 nm and smaller than or equal to 10 μm. Thegraphene compound sheet may be modified with an atom other than carbon,an atomic group containing an atom other than carbon, an atomic groupcomposed mainly of carbon such as an alkyl group, or the like. Aplurality of layers in the graphene compound sheet may be modified withdifferent atoms or atomic groups.

A graphene compound may have a five-membered ring composed of carbonatoms or a poly-membered ring that is a seven- or more-membered ringcomposed of carbon atoms, in addition to a six-membered ring composed ofcarbon atoms. In the neighborhood of a poly-membered ring which is aseven- or more-membered ring, a region through which a lithium ion canpass may be generated.

A plurality of graphene compounds may be gathered to form a sheet-likeshape. A graphene compound has a planar shape, thereby enabling surfacecontact.

In some cases, a graphene compound has high conductivity even when it isthin. The contact area between graphene compounds or between a graphenecompound and an active material can be increased by surface contact.Thus, even with a small amount of a graphene compound per volume, aconductive path can be formed efficiently.

In contrast, a graphene compound may also be used as an insulator. Forexample, a graphene compound sheet may be used as a sheet-likeinsulator. Graphene oxide, for example, has a higher insulation propertythan a graphene compound that is not oxidized, in some cases. A graphenecompound modified with an atomic group may have an improved insulationproperty, depending on the type of the modifying atomic group.

A graphene compound in this specification and the like may include aprecursor of graphene. The precursor of graphene refers to a substanceused for forming graphene. The precursor of graphene may contain theabove-described graphene oxide, graphite oxide, or the like.

Graphene containing an alkali metal or an element other than carbon,such as oxygen, is referred to as a graphene analog in some cases. Inthis specification and the like, graphene compounds include grapheneanalogs.

A graphene compound in this specification and the like may include anatom, an atomic group, and ions of them between the layers. The physicalproperties, such as electric conductivity and ion conductivity, of agraphene compound sometimes change when an atom, an atomic group, andions of them exist between layers of the compound. For example, bymixing a lithium salt with a graphene compound, the ion conductivity ofthe graphene compound can be increased. As the lithium salt, one or moreof LiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiAlCl₄, LiSCN, LiBr, LiI, Li₂SO₄,Li₂B₁₀Cl₁₀, Li₂B₁₂Cl₁₂, LiCF₃SO₃, LiC₄F₉SO₃, LiC(CF₃SO₂)₃,LiC(C₂F₅SO₂)₃, LiN(CF₃SO₂)₂, LiN(C₄F₉SO₂) (CF₃SO₂), LiN(C₂F₅SO₂)₂, andthe like can be used. In addition, a distance between the layers isincreased in some cases.

A graphene compound has excellent electrical characteristics of highconductivity and excellent physical properties of high flexibility andhigh mechanical strength in some cases. A modified graphene compound canhave extremely low conductivity and serve as an insulator depending onthe type of the modification. A graphene compound has a planar shape. AGraphene compound enables low-resistance surface contact.

[3. Positive Electrode]

The positive electrode 101 includes, for example, a positive electrodecurrent collector 101 a and a positive electrode active material layer101 b formed on the positive electrode current collector 101 a. In thisembodiment, an example of providing the positive electrode activematerial layer 101 b on one surface of the positive electrode currentcollector 101 a having a sheet shape (or a strip-like shape) is given.However, this embodiment is not limited thereto; the positive electrodeactive material layer 101 b may be provided on both surfaces of thepositive electrode current collector 101 a. Providing the positiveelectrode active material layer 101 b on both surfaces of the positiveelectrode current collector 101 a allows the lithium-ion storage battery100 to have high capacity. Furthermore, in this embodiment, the positiveelectrode active material layer 101 b is provided on the whole positiveelectrode current collector 101 a. However, this embodiment is notlimited thereto; the positive electrode active material layer 101 b maybe provided on a part of the positive electrode current collector 101 a.For example, the positive electrode active material layer 101 b is notprovided on a portion of the positive electrode current collector 101 awhich is to be electrically in contact with the positive electrode lead104 (hereinafter, the portion is also referred to as a “positiveelectrode tab”).

The positive electrode current collector 101 a can be formed using amaterial that has high conductivity and is not alloyed with a carrierion of lithium or the like, such as stainless steel, gold, platinum,zinc, iron, copper, aluminum, or titanium, an alloy thereof, or thelike. Alternatively, an aluminum alloy to which an element whichimproves heat resistance, such as silicon, titanium, neodymium,scandium, or molybdenum, is added can be used. Still alternatively, ametal element which forms silicide by reacting with silicon can be used.Examples of the metal element which forms silicide by reacting withsilicon include zirconium, titanium, hafnium, vanadium, niobium,tantalum, chromium, molybdenum, tungsten, cobalt, nickel, and the like.The positive electrode current collector 101 a can have a foil-likeshape, a plate-like shape (sheet-like shape), a net-like shape, apunching-metal shape, an expanded-metal shape, or the like asappropriate. The positive electrode current collector 101 a preferablyhas a thickness greater than or equal to 5 μm and less than or equal to30 μm. The surface of the positive electrode current collector 101 a maybe provided with an undercoat using graphite or the like.

The positive electrode active material layer 101 b may further include abinder for increasing adhesion of positive electrode active materials, aconductive additive for increasing the conductivity of the positiveelectrode active material layer 101 b, and the like in addition to thepositive electrode active material.

Examples of a positive electrode active material used for the positiveelectrode active material layer 101 b include a composite oxide with anolivine crystal structure, a composite oxide with a layered rock-saltcrystal structure, and a composite oxide with a spinel crystalstructure. As the positive electrode active material, a compound such asLiFeO₂, LiCoO₂, LiNiO₂, LiMn₂O₄, V₂O₅, Cr₂O₅, and MnO₂ is used. LiCoO₂is particularly preferable because it has high capacity, stability inthe air higher than that of LiNiO₂, and thermal stability higher thanthat of LiNiO₂, for example.

It is preferable to add a small amount of lithium nickel oxide (LiNiO₂or LiNi_(1-x)M_(x)O₂ (0<x<1, M=Co, Al, or the like)) to alithium-containing material with a spinel crystal structure whichcontains manganese such as LiMn₂O₄ because the dissolution of manganeseand the decomposition of an electrolyte solution can be suppressed, forexample.

Alternatively, a complex material (LiMPO₄ (general formula) (M is one ormore of Fe(II), Mn(II), Co(II), and Ni(II))) can be used. Typicalexamples of the general formula LiMPO₄ are lithium compounds such asLiFePO₄, LiNiPO₄, LiCoPO₄, LiMnPO₄, LiFe_(a)Ni_(b)PO₄,LiFe_(a)Co_(b)PO₄, LiFe_(a)Mn_(b)PO₄, LiNi_(a)Co_(b)PO₄,LiNi_(a)Mn_(b)PO₄ (a+b≤1, 0<a<1, and 0<b<1), LiFe_(c)Ni_(d)Co_(e)PO₄,LiFe_(c)Ni_(d)Mn_(e)PO₄, LiNi_(c)Co_(d)Mn_(e)PO₄ (c+d+e≤1, 0<c<1, 0<d<1,and 0<e<1), and LiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≤1, 0<f<1, 0<g<1,0<h<1, and 0<i<1).

LiFePO₄ is particularly preferable because it properly satisfiesconditions necessary for the positive electrode active material, such assafety, stability, high capacity density, high potential, and theexistence of lithium ions which can be extracted in initial oxidation(charging).

Alternatively, a complex material such as Li_((2-j))MSiO₄ (generalformula) (M is one or more of Fe(II), Mn(II), Co(II), and Ni(II); 0≤j≤2)may be used. Typical examples of the general formula Li_((2-j))MSiO₄ arelithium compounds such as Li_((2-j))FeSiO₄, Li_((2-j))NiSiO₄,Li_((2-j))CoSiO₄, Li_((2-j))MnSiO₄, Li_((2-j))Fe_(k)Ni_(l)SiO₄,Li_((2-j))Fe_(k)Co_(l)SiO₄, Li_((2-j))Fe_(k)Mn_(l)SiO₄,Li_((2-j))Ni_(k)Co_(l)SiO₄, Li_((2-j))Ni_(k)Mn_(l)SiO₄ (k+l≤1, 0<k<1,and 0<l<1), Li_((2-j))Fe_(m)Ni_(n)Co_(q)SiO₄,Li_((2-j))Fe_(m)Ni_(n)Mn_(q)SiO₄, Li_((2-j))Ni_(m)Co_(n)Mn_(q)SiO₄(m+n+q≤1, 0<m<1, 0<n<1, and 0<q<1), andLi_((2-j))Fe_(r)Ni_(s)Co_(r)Mn_(u)SiO₄ (r+s+t+u≤1, 0<r<1, 0<s<1, 0<t<1,and <u<1).

Still alternatively, a nasicon compound expressed by A_(x)M₂(XO₄)₃(general formula) (A=Li, Na, or Mg, M=Fe, Mn, Ti, V, Nb, or Al, X=S, P,Mo, W, As, or Si) can be used as the positive electrode active material.Examples of the nasicon compound are Fe₂(MnO₄)₃, Fe₂(SO₄)₃, andLi₃Fe₂(PO₄)₃. Still further alternatively, compounds represented by ageneral formula, Li₂MPO₄F, Li₂MP₂O₇, and Li₅MO₄ (M=Fe or Mn), aperovskite fluoride such as NaFeF₃ and FeF₃, a metal chalcogenide (asulfide, a selenide, and a telluride) such as TiS₂ and MoS₂, an oxidewith an inverse spinel crystal structure such as LiMVO₄, a vanadiumoxide (e.g., V₂O₅, V₆O₁₃, and LiV₃O₈), a manganese oxide, and an organicsulfur compound can be used as the positive electrode active material,for example.

In the case where carrier ions are alkali metal ions other than lithiumions or alkaline-earth metal ions, the positive electrode activematerial may contain, instead of lithium, an alkali metal (e.g., sodiumor potassium) or an alkaline-earth metal (e.g., calcium, strontium,barium, beryllium, or magnesium). For example, the positive electrodeactive material may be a layered oxide containing sodium such as NaFeO₂or Na_(2/3)[Fe_(1/2)Mn_(1/2)]O₂.

Further alternatively, any of the aforementioned materials may becombined to be used as the positive electrode active material. Forexample, the positive electrode active material may be a solid solutioncontaining any of the aforementioned materials, e.g., a solid solutioncontaining LiCo_(1/3)Mn_(1/3)Ni_(1/3)O₂ and Li₂MnO₃.

Note that although not shown, a conductive material such as a carbonlayer may be provided on a surface of the positive electrode activematerial layer 101 b. With the conductive material such as the carbonlayer, conductivity of the electrode can be increased. For example, thepositive electrode active material layer 101 b can be coated with thecarbon layer by mixing a carbohydrate such as glucose at the time ofbaking the positive electrode active material.

The average particle diameter of the primary particle of the positiveelectrode active material layer 101 b is preferably greater than orequal to 50 nm and less than or equal to 100 μm.

Examples of the conductive additive include acetylene black (AB),graphite (black lead) particles, carbon nanotubes, a graphene compound,and fullerene.

A network for electron conduction can be formed in the positiveelectrode 101 by the conductive additive. The conductive additive alsoallows maintaining of a path for electric conduction between theparticles of the positive electrode active material layer 101 b. Theaddition of the conductive additive to the positive electrode activematerial layer 101 b increases the electron conductivity of the positiveelectrode active material layer 101 b.

As the binder, instead of polyvinylidene fluoride (PVDF) as a typicalone, polyimide, polytetrafluoroethylene, polyvinyl chloride,ethylene-propylene-diene polymer, styrene-butadiene rubber,acrylonitrile-butadiene rubber, fluorine rubber, polyvinyl acetate,polymethyl methacrylate, polyethylene, nitrocellulose or the like can beused.

The content of the binder in the positive electrode active materiallayer 101 b is preferably greater than or equal to 1 wt % and less thanor equal to 10 wt %, more preferably greater than or equal to 2 wt % andless than or equal to 8 wt %, and still more preferably greater than orequal to 3 wt % and less than or equal to 5 wt %. The content of theconductive additive in the positive electrode active material layer 101b is preferably greater than or equal to 1 wt % and less than or equalto 10 wt %, more preferably greater than or equal to 1 wt % and lessthan or equal to 5 wt %.

In the case where the positive electrode active material layer 101 b isformed by a coating method, the positive electrode active material, thebinder, and the conductive additive are mixed to form a positiveelectrode paste (slurry), and the positive electrode paste is applied tothe positive electrode current collector 101 a and dried.

The positive electrode active material layer 101 b may be formed by asputtering method.

In the case where the positive electrode is wrapped in the grapheneoxide film 103, the graphene oxide film 103 for wrapping the positiveelectrode may be formed by a cast method. Schematic cross-sectionalviews of the positive electrode and the graphene oxide film that areformed in this manner are similar to FIGS. 30A to 30C, i.e., schematiccross-sectional views illustrating the case where the negative electrodeis wrapped in the graphene oxide film.

[4. Negative Electrode]

The negative electrode 102 includes, for example, a negative electrodecurrent collector 102 a and a negative electrode active material layer102 b formed on the negative electrode current collector 102 a. In thisembodiment, an example of providing the negative electrode activematerial layer 102 b on one surface of the negative electrode currentcollector 102 a having a sheet shape (or a strip-like shape) is given.However, this embodiment is not limited thereto; the negative electrodeactive material layer 102 b may be provided on both surfaces of thenegative electrode current collector 102 a. Providing the negativeelectrode active material layer 102 b on both surfaces of the negativeelectrode current collector 102 a allows the lithium-ion storage battery100 to have high capacity. Furthermore, in this embodiment, the negativeelectrode active material layer 102 b is provided on the whole negativeelectrode current collector 102 a. However, this embodiment is notlimited thereto; the negative electrode active material layer 102 b maybe provided on a part of the negative electrode current collector 102 a.For example, the negative electrode active material layer 102 b is notprovided on a portion of the negative electrode current collector 102 awhich is to be electrically in contact with the negative electrode lead105 (hereinafter, the portion is also referred to as a “negativeelectrode tab”).

The negative electrode current collector 102 a can be formed using amaterial that has high conductivity and is not alloyed with a carrierion of lithium or the like, such as stainless steel, gold, platinum,zinc, iron, copper, titanium, an alloy thereof, or the like.Alternatively, a metal element which forms silicide by reacting withsilicon can be used. Examples of the metal element which forms silicideby reacting with silicon include zirconium, titanium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, andthe like. The negative electrode current collector 102 a can have afoil-like shape, a plate-like shape (sheet-like shape), a net-likeshape, a punching-metal shape, an expanded-metal shape, or the like asappropriate. The negative electrode current collector 102 a preferablyhas a thickness greater than or equal to 5 μm and less than or equal to30 μm. The surface of the negative electrode current collector 102 a maybe provided with an undercoat using graphite or the like.

The negative electrode active material layer 102 b may further include abinder for increasing adhesion of negative electrode active materials, aconductive additive for increasing the conductivity of the negativeelectrode active material layer 102 b, and the like in addition to thenegative electrode active materials.

There is no particular limitation on the material of the negativeelectrode active material layer 102 b as long as it is a material withwhich lithium can be dissolved and precipitated or a material into/fromwhich lithium ions can be inserted and extracted. Other than a lithiummetal or lithium titanate, a carbon-based material generally used in thefield of power storage, or an alloy-based material can also be used asthe negative electrode active material layer 102 b.

The lithium metal is preferable because of its low redox potential(3.045 V lower than that of a standard hydrogen electrode) and highspecific capacity per unit weight and per unit volume (3860 mAh/g and2062 mAh/cm³).

Examples of the carbon-based material include graphite, graphitizingcarbon (soft carbon), non-graphitizing carbon (hard carbon), a carbonnanotube, a graphene compound, carbon black, and the like.

Examples of the graphite include artificial graphite such as meso-carbonmicrobeads (MCMB), coke-based artificial graphite, or pitch-basedartificial graphite and natural graphite such as spherical naturalgraphite.

Graphite has a low potential substantially equal to that of a lithiummetal (0.1 V to 0.3 V vs. Li/Li⁺) when lithium ions are inserted intothe graphite (when a lithium-graphite intercalation compound is formed).For this reason, a lithium ion battery can have a high operatingvoltage. In addition, graphite is preferable because of its advantagessuch as relatively high capacity per unit volume, small volumeexpansion, low cost, and safety greater than that of a lithium metal.

For the negative electrode active material, an alloy-based materialwhich enables charge-discharge reaction by an alloying reaction and adealloying reaction with lithium can be used. In the case where lithiumions are carrier ions, the alloy-based material is, for example, amaterial containing at least one of Al, Si, Ge, Sn, Pb, Sb, Bi, Ag, Zn,Cd, In, Ga, and the like. Such elements have higher capacity thancarbon. In particular, silicon has a theoretical capacity of 4200 mAh/g,which is significantly high. For this reason, silicon is preferably usedas the negative electrode active material. Examples of the alloy-basedmaterial using such elements include Mg₂Si, Mg₂Ge, Mg₂Sn, SnS₂, V₂Sn₃,FeSn₂, CoSn₂, Ni₃Sn₂, Cu₆Sn₅, Ag₃Sn, Ag₃Sb, Ni₂MnSb, CeSb₃, LaSn₃,La₃Co₂Sn₇, CoSb₃, InSb, SbSn, and the like.

Alternatively, as the negative electrode active material layer 102 b,oxide such as SiO, SnO, SnO₂, titanium oxide (TiO₂), lithium titaniumoxide (Li₄Ti₅O₁₂), lithium-graphite intercalation compound (LixC₆),niobium oxide (Nb₂O₅), tungsten oxide (WO₂), molybdenum oxide (MoO₂), orthe like can be used.

Still alternatively, as the negative electrode active material layer 102b, (M=Co, Ni, or Cu) with a Li₃N structure, which is a nitridecontaining lithium and a transition metal, can be used. For example,Li_(2.6)Co_(0.4)N₃ is preferable because of high charge and dischargecapacity (900 mAh/g and 1890 mAh/cm³).

A nitride containing lithium and a transition metal is preferably used,in which case lithium ions are contained in the negative electrodeactive material and thus the negative electrode active material can beused in combination with a material for a positive electrode activematerial which does not contain lithium ions, such as V₂O₅ or Cr₃O₈.Note that in the case of using a material containing lithium ions as apositive electrode active material, the nitride containing lithium and atransition metal can be used as the negative electrode active materialby extracting the lithium ions contained in the positive electrodeactive material in advance.

Still further alternatively, as the negative electrode active materiallayer 102 b, a material which causes conversion reaction can be used.For example, a transition metal oxide with which an alloying reactionwith lithium is not caused, such as cobalt oxide (CoO), nickel oxide(NiO), or iron oxide (FeO), may be used for the negative electrodeactive material. Other examples of the material which causes aconversion reaction include oxides such as Fe₂O₃, CuO, Cu₂O, RuO₂, andCr₂O₃, sulfides such as CoS_(0.89), NiS, or CuS, nitrides such as Zn₃N₂,Cu₃N, and Ge₃N₄, phosphides such as NiP₂, FeP₂, and CoP₃, and fluoridessuch as FeF₃ and BiF₃. Note that any of the fluorides can be used as thenegative electrode active material layer 102 b because of its highpotential.

In the case where the negative electrode active material layer 102 b isformed by a coating method, the negative electrode active material andthe binder are mixed to form a negative electrode paste (slurry), andthe negative electrode paste is applied to the negative electrodecurrent collector 102 a and dried. Note that a conductive additive maybe added to the negative electrode paste. The negative electrode activematerial layer 102 b may be formed by a sputtering method.

Then, the graphene oxide film 103 that wraps the negative electrodeactive material layer may be formed by a cast method. Examples of across-sectional structure of the negative electrode and the grapheneoxide film in that case are shown in FIGS. 30A to 30C.

A graphene compound may be formed on a surface of the negative electrodeactive material layer 102 b. For example, in the case of using siliconas the negative electrode active material layer 102 b, the volume ofsilicon is greatly changed due to occlusion and release of carrier ionsin charge-discharge cycles. Thus, adhesion between the negativeelectrode current collector 102 a and the negative electrode activematerial layer 102 b is decreased, resulting in degradation of batterycharacteristics caused by charge and discharge. In view of this, agraphene compound is preferably formed on a surface of the negativeelectrode active material layer 102 b containing silicon because evenwhen the volume of silicon is changed in charge-discharge cycles,separation between the negative electrode current collector 102 a andthe negative electrode active material layer 102 b can be prevented,which makes it possible to reduce degradation of batterycharacteristics.

Further, a coating film of oxide or the like may be formed on thesurface of the negative electrode active material layer 102 b. A coatingfilm formed by decomposition or the like of an electrolyte solution orthe like in charging cannot release electric charges used at theformation, and therefore forms irreversible capacity. In contrast, thefilm of an oxide or the like provided on the surface of the negativeelectrode active material layer 102 b in advance can reduce or preventgeneration of irreversible capacity.

As the coating film coating the negative electrode active material layer102 b, an oxide film of any one of niobium, titanium, vanadium,tantalum, tungsten, zirconium, molybdenum, hafnium, chromium, aluminum,and silicon or an oxide film containing any one of these elements andlithium can be used. Such a film is denser than a conventional filmformed on a surface of a negative electrode due to a decompositionproduct of an electrolyte solution.

For example, niobium pentoxide (Nb₂O₅) has a low electric conductivityof 10⁻⁹ S/cm and a high insulating property. For this reason, a niobiumoxide film inhibits electrochemical decomposition reaction between thenegative electrode active material and the electrolyte solution. On theother hand, niobium oxide has a lithium diffusion coefficient of 10⁻⁹cm²/sec and high lithium ion conductivity. Therefore, niobium oxide cantransmit lithium ions. Alternatively, silicon oxide or aluminum oxidemay be used.

A sol-gel method can be used to coat the negative electrode activematerial layer 102 b with the coating film, for example. The sol-gelmethod is a method for forming a thin film in such a manner that asolution of metal alkoxide, a metal salt, or the like is changed into agel, which has lost its fluidity, by hydrolysis reaction andpolycondensation reaction and the gel is baked. Since a thin film isformed from a liquid phase in the sol-gel method, raw materials can bemixed uniformly on the molecular scale. For this reason, by adding anegative electrode active material such as graphite to a raw material ofthe metal oxide film which is a solvent, the active material can beeasily dispersed into the gel. In such a manner, the coating film can beformed on the surface of the negative electrode active material layer102 b. A decrease in the capacity of the storage battery can beprevented by using the coating film.

[5. Electrolyte Solution]

As a solvent of the electrolyte solution 106 used for the storagebattery 100, an aprotic organic solvent is preferably used. For example,one of ethylene carbonate (EC), propylene carbonate (PC), butylenecarbonate, chloroethylene carbonate, vinylene carbonate,γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethylcarbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methylacetate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane(DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile,benzonitrile, tetrahydrofuran, sulfolane, and sultone can be used, ortwo or more of these solvents can be used in an appropriate combinationin an appropriate ratio.

When a gelled high-molecular material is used as the solvent for theelectrolyte solution, safety against liquid leakage and the like isimproved. Further, a secondary battery can be thinner and morelightweight. Typical examples of the gelled high-molecular materialinclude a silicone gel, an acrylic gel, an acrylonitrile gel, apoly(ethylene oxide)-based gel, a poly(propylene oxide)-based gel, a gelof a fluorine-based polymer, and the like.

Alternatively, the use of one or more ionic liquids (room temperaturemolten salts) which are less likely to burn and volatilize as thesolvent of the electrolyte solution can prevent the storage battery fromexploding or catching fire even when the storage battery internallyshorts out or the internal temperature increases due to overcharging orthe like.

In the case of using a lithium ion as a carrier ion, as an electrolytedissolved in the above-described solvent, one of lithium salts such asLiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiAlCl₄, LiSCN, LiBr, LiI, Li₂SO₄,Li₂B₁₀Cl₁₀, Li₂B₁₂Cl₁₂, LiCF₃SO₃, LiC₄F₉SO₃, LiC(CF₃SO₂)₃,LiC(C₂F₅SO₂)₃, LiN(CF₃SO₂)₂, LiN(C₄F₉SO₂) (CF₃SO₂), and LiN(C₂F₅SO₂)₂can be used, or two or more of these lithium salts can be used in anappropriate combination in an appropriate ratio.

The electrolyte solution used for the storage battery preferablycontains a small amount of dust particles and elements other than theconstituent elements of the electrolyte solution (hereinafter, alsosimply referred to as impurities) so as to be highly purified.Specifically, the weight ratio of impurities to the electrolyte solutionis less than or equal to 1%, preferably less than or equal to 0.1%, andmore preferably less than or equal to 0.01%. An additive agent such asvinylene carbonate may be added to the electrolyte solution.

[6. Exterior Body]

The secondary battery can have any of a variety of structures. In thisembodiment, a film is used for the exterior body 107. Note that the filmused for the exterior body 107 is a single-layer film selected from ametal film (e.g., an aluminum film, a stainless steel film, and a nickelsteel film), a plastic film made of an organic material, a hybridmaterial film including an organic material (e.g., an organic resin orfiber) and an inorganic material (e.g., ceramic), and acarbon-containing inorganic film (e.g., a carbon film or a graphitefilm); or a stacked-layer film including two or more of the above films.A metal film is easily embossed. Forming a depression or a projection ona surface of a metal film by embossing increases the surface area of theexterior body 107 exposed to outside air, achieving efficient heatdissipations.

In the case where the lithium-ion storage battery 100 is changed in formby externally applying force, bending stress is externally applied tothe exterior body 107 of the lithium-ion storage battery 100. This mightpartly deform or damage the exterior body 107. The depression orprojection formed on the surface of the exterior body 107 can relieve astrain caused by stress applied to the exterior body 107. Therefore, thelithium-ion storage battery 100 can have high reliability. Note that a“strain” is the scale of change in form indicating the displacement of apoint of an object relative to the reference (initial) length of theobject. The depression or the projection formed on the surface of theexterior body 107 can reduce the influence of a strain caused byapplication of external force to the lithium-ion storage battery to anacceptable level. Thus, the lithium-ion storage battery having highreliability can be provided.

[7. Separator]

In this embodiment, the separator 109 may be provided between thepositive electrode 101 and the negative electrode 102 as shown in FIG.1A, FIG. 2A, and FIG. 3A, for example.

As the separator 109, paper, nonwoven fabric, a glass fiber, a syntheticfiber such as nylon (polyamide), vinylon (also called vinalon) (apolyvinyl alcohol based fiber), polyester, acrylic, polyolefin, orpolyurethane, or the like may be used. The above-described graphenecompound may be used as well. However, a material which does notdissolve in an electrolyte solution should be selected.

More specific examples of materials of the separator 109 arehigh-molecular compounds based on fluorine-based polymer, polyether suchas polyethylene oxide and polypropylene oxide, polyolefin such aspolyethylene and polypropylene, polyacrylonitrile, polyvinylidenechloride, polymethyl methacrylate, polymethylacrylate, polyvinylalcohol, polymethacrylonitrile, polyvinyl acetate, polyvinylpyrrolidone,polyethyleneimine, polybutadiene, polystyrene, polyisoprene, and apolyurethane-based polymer, derivatives thereof, cellulose, paper, andnonwoven fabric, which can be used either alone or in combination.

In the case where the graphene oxide film 103 can have a function of aseparator, the separator 109 need not be provided additionally, but oneembodiment of the present invention is not limited thereto. When theseparator 109 is provided additionally, the lithium-ion storage batteryof one embodiment of the present invention including the separator 109can be driven more safely in some cases.

[8. Assembly of Storage Battery and Aging]

The above-described components are combined and sealed in the exteriorbody 107. Thus, as shown in FIGS. 1A and 1B, FIGS. 2A and 2B, and FIGS.3A and 3B, the positive electrode 101, the negative electrode 102, andthe graphene oxide film 103 are stacked and sealed in the exterior body107 together with the electrolyte solution 106. FIGS. 6A and 6B show aprocess of storing the components in the exterior body 107. In the casewhere the separator 109 (not shown in FIGS. 6A and 6B) is used, theseparator 109 is placed between the positive electrode 101 and thenegative electrode 102.

Note that a sealing portion may have a shape with a curve, a shape witha wavy line, a shape with an arch, or a shape with a plurality ofinflection points along the shape of an inner structure of the storagebattery. In the case where the shape of the sealing portion isdetermined according to the shape of the inner structure, even when thestorage battery is deformed and different stresses are applied to theexterior body and the inner structure, an unintentionally large amountof sliding of the inner structure can be prevented. However, the shapeof the sealing portion is not limited thereto.

Then, an aging step is performed. First, the environmental temperatureis kept at about room temperature for example, and constant currentcharge is performed to a predetermined voltage at a low rate. Next, agas generated in a region inside the exterior body by charging isreleased outside the exterior body, and then charge is performed at arate higher than that of the initial charge.

After that, the storage battery is preserved at relatively hightemperatures for a long time. For example, the storage battery is keptat higher than or equal to 40° C. for longer than or equal to 24 hours.It may be preserved at higher than or equal to 70° C., for example. Itmay be preserved at higher than or equal to 80° C. It may be preservedat higher than or equal to 90° C. It may be preserved at higher than orequal to 100° C. It may be preserved for longer than or equal to 36hours. It may be preserved for longer than or equal to 48 hours. It maybe preserved for longer than or equal to 72 hours. Keeping the storagebattery at an environmental temperature that is high but not so high asto cause danger can sufficiently serve as an aging step and contributeto less degradation of the storage battery in some cases. Also in thecase where the preservation time is long but not so long as to causedanger, keeping the storage battery can sufficiently serve as an agingstep and contribute to less degradation of the storage battery in somecases.

After the storage battery is preserved at relatively high temperaturesfor a long time, gases generated in a region inside the exterior body isreleased again. Furthermore, at room temperature, the storage battery isdischarged at a rate of 0.2 C, charged at the same rate, discharged atthe same rate again, and further charged at the same rate. Then,discharge is performed at the same rate, which terminates the agingstep.

In the aforementioned manner, the lithium-ion storage battery of oneembodiment of the present invention can be fabricated.

This embodiment can be implemented in appropriate combination with theother embodiments.

Note that in the case where at least one specific example is describedin a diagram or a text described in one embodiment in this specificationand the like, it will be readily appreciated by those skilled in the artthat a broader concept of the specific example can be derived.Therefore, in the diagram or the text described in one embodiment, inthe case where at least one specific example is described, a broaderconcept of the specific example is disclosed as one embodiment of theinvention, and one embodiment of the invention can be constituted. Theembodiment of the present invention is clear.

Note that in this specification and the like, what is illustrated in atleast a diagram (which may be part of the diagram) is disclosed as oneembodiment of the invention, and one embodiment of the invention can beconstituted. Therefore, when certain contents are described in adiagram, the contents are disclosed as one embodiment of the inventioneven when the contents are not described with text, and one embodimentof the invention can be constituted. In a similar manner, part of adiagram, which is taken out from the diagram, is disclosed as oneembodiment of the invention, and one embodiment of the invention can beconstituted. The embodiment of the present invention is clear.

In Embodiment 1, one embodiment of the present invention has beendescribed. Other embodiments of the present invention are described inEmbodiments 2 to 4. Note that one embodiment of the present invention isnot limited thereto. In other words, various embodiments of theinvention are described in this embodiment and the other embodiments,and one embodiment of the present invention is not limited to aparticular embodiment. For example, although an example in which oneembodiment of the present invention is applied to a lithium-ion storagebattery is described, one embodiment of the present invention is notlimited thereto. Depending on circumstances or conditions, applicationof one embodiment of the present invention to a variety of secondarybatteries such as a lead storage battery, a lithium-ion polymersecondary battery, a nickel-hydrogen storage battery, a nickel-cadmiumstorage battery, a nickel-iron storage battery, a nickel-zinc storagebattery, a silver oxide-zinc storage battery, a solid-state battery, andan air battery; a primary battery; a capacitor such as an electricdouble layer capacitor, an ultracapacitor, a supercapacitor, and alithium ion capacitor; and the like is also possible. Furthermore,depending on circumstances or conditions, the graphene oxide film 103 isnot necessarily used in one embodiment of the present invention.

Embodiment 2

In this embodiment, a flexible lithium-ion storage battery is described.

<<Flexible Storage Battery>>

When a flexible material is selected from materials of the membersdescribed in this embodiment and used, a flexible lithium-ion storagebattery can be fabricated. Deformable devices are currently under activeresearch and development. For such devices, flexible storage batteriesare demanded.

Deformation of a storage battery is described with reference to FIGS. 7Ato 7D. In the case of bending a storage battery in which a batterymaterial 1805 including electrodes and an electrolyte solution issandwiched between two films as exterior bodies, a radius 1802 ofcurvature of a film 1801 closer to a center 1800 of curvature of thestorage battery is smaller than a radius 1804 of curvature of a film1803 far from the center 1800 of curvature (FIG. 7A). When the storagebattery is curved and has an arc-shaped cross section, compressivestress is applied to a surface of the film on the side closer to thecenter 1800 of curvature and tensile stress is applied to a surface ofthe film on the side farther from the center 1800 of curvature (FIG.7B).

When a flexible lithium-ion storage battery is deformed, strong stressis applied to the exterior bodies. However, even with the compressivestress and tensile stress due to the deformation of the storage battery,the influence of a strain can be reduced by forming a pattern includingprojections or depressions on surfaces of the exterior bodies. For thisreason, the storage battery can change its form in such a range that theexterior body on the side closer to the center of curvature has acurvature radius of 50 mm or less, or a curvature radius of 30 mm orless.

The radius of curvature of a surface is described with reference todrawings. In FIG. 8A, on a plane 1701 along which a curved surface 1700is cut, part of a curve 1702 forming the curved surface 1700 isapproximate to an arc of a circle; the radius of the circle is referredto as a radius of curvature 1703 and the center of the circle isreferred to as a center of curvature 1704. FIG. 8B is a top view of thecurved surface 1700. FIG. 8C is a cross-sectional view of the curvedsurface 1700 taken along the plane 1701. When a curved surface is cut bya plane, the radius of curvature of a curve in a cross section differsdepending on the angle between the curved surface and the plane or onthe cut position, and the smallest radius of curvature is defined as theradius of curvature of a surface in this specification and the like.

Note that the cross-sectional shape of the storage battery is notlimited to a simple arc shape, and the cross section can be partlyarc-shaped; for example, a shape illustrated in FIG. 7C, a wavy shapeillustrated in FIG. 7D, or an S shape can be used. When the curvedsurface of the storage battery has a shape with a plurality of centersof curvature, the storage battery can change its form in such a rangethat a curved surface with the smallest radius of curvature among radiiof curvature with respect to the plurality of centers of curvature,which is a surface of the exterior body on the side closer to the centerof curvature, can have a curvature radius of 50 mm, or a curvatureradius of 30 mm in some cases.

This embodiment can be implemented in appropriate combination with theother embodiments.

Embodiment 3

In this embodiment, structures of a storage battery of one embodiment ofthe present invention are described with reference to FIGS. 9A to 9C,FIGS. 10A and 10B, and FIGS. 11A and 11B.

<<Coin-Type Storage Battery>>

FIG. 9A is an external view of a coin-type (single-layer flat type)storage battery, and FIG. 9B is a cross-sectional view thereof.

In a coin-type storage battery 300, a positive electrode can 301doubling as a positive electrode terminal and a negative electrode can302 doubling as a negative electrode terminal are insulated from eachother and sealed by a gasket 303 made of polypropylene or the like. Apositive electrode 304 includes a positive electrode current collector305 and a positive electrode active material layer 306 provided incontact with the positive electrode current collector 305. The positiveelectrode active material layer 306 may further include a binder forincreasing adhesion of positive electrode active materials, a conductiveadditive for increasing the conductivity of the positive electrodeactive material layer, and the like in addition to the positiveelectrode active materials.

A negative electrode 307 includes a negative electrode current collector308 and a negative electrode active material layer 309 provided incontact with the negative electrode current collector 308. The negativeelectrode active material layer 309 may further include a binder forincreasing adhesion of negative electrode active materials, a conductiveadditive for increasing the conductivity of the negative electrodeactive material layer, and the like in addition to the negativeelectrode active materials. A separator 310 and an electrolyte (notillustrated) are provided between the positive electrode active materiallayer 306 and the negative electrode active material layer 309. At leastone of the positive electrode 304 and the negative electrode 307 iswrapped in a graphene oxide film (not illustrated).

The materials described in Embodiment 1 can be used for the components.

For the positive electrode can 301 and the negative electrode can 302, ametal having a corrosion-resistant property to an electrolyte solution,such as nickel or titanium, an alloy of such a metal, or an alloy ofsuch a metal and another metal (e.g., stainless steel or the like) canbe used. Alternatively, the positive electrode can 301 and the negativeelectrode can 302 are preferably covered with nickel or the like inorder to inhibit corrosion due to the electrolyte solution. The positiveelectrode can 301 and the negative electrode can 302 are electricallyconnected to the positive electrode 304 and the negative electrode 307,respectively.

The negative electrode 307, the positive electrode 304, and theseparator 310 are immersed in the electrolyte. Then, as illustrated inFIG. 9B, the positive electrode 304, the separator 310, the negativeelectrode 307, and the negative electrode can 302 are stacked in thisorder with the positive electrode can 301 positioned at the bottom, andthe positive electrode can 301 and the negative electrode can 302 aresubjected to pressure bonding with the gasket 303 interposedtherebetween. In such a manner, the coin-type storage battery 300 can befabricated.

Here, a current flow in charging a storage battery will be describedwith reference to FIG. 9C. When a storage battery using lithium isregarded as a closed circuit, lithium ions transfer and a current flowsin the same direction. Note that in the storage battery using lithium,an anode and a cathode change places in charge and discharge, and anoxidation reaction and a reduction reaction occur on the correspondingsides; hence, an electrode with a high redox potential is called apositive electrode and an electrode with a low redox potential is calleda negative electrode. For this reason, in this specification, thepositive electrode is referred to as a “positive electrode” or a “pluselectrode” and the negative electrode is referred to as a “negativeelectrode” or a “minus electrode” in all the cases where charge isperformed, discharge is performed, a reverse pulse current is supplied,and a charging current is supplied. The use of the terms “anode” and“cathode” related to an oxidation reaction and a reduction reactionmight cause confusion because the anode and the cathode change places atthe time of charging and discharging. Thus, the terms “anode” and“cathode” are not used in this specification. If the term “anode” or“cathode” is used, it should be mentioned that the anode or the cathodeis which of the one at the time of charging or the one at the time ofdischarging and corresponds to which of a positive electrode or anegative electrode.

Two terminals shown in FIG. 9C are connected to a charger, and a storagebattery 400 is charged. As the charge of the storage battery 400proceeds, a potential difference between electrodes increases. Thepositive direction in FIG. 9C is the direction in which a current flowsfrom one terminal (a tab electrode) outside the storage battery 400 to apositive electrode 402, flows from the positive electrode 402 to anegative electrode 404 through an electrolyte solution 406 in thestorage battery 400 and a separator 408 in the electrolyte solution 406,and flows from the negative electrode to the other terminal (a tabelectrode) outside the storage battery 400. In other words, a directionin which a charging current flows is regarded as a direction of acurrent.

5<<Cylindrical Storage Battery>>

Next, an example of a cylindrical storage battery will be described withreference to FIGS. 10A and 10B. As illustrated in FIG. 10A, acylindrical storage battery 600 includes a positive electrode cap(battery cap) 601 on the top surface and a battery can (outer can) 602on the side surface and bottom surface. The positive electrode cap andthe battery can 602 are insulated from each other by a gasket(insulating gasket) 610.

FIG. 10B is a diagram schematically illustrating a cross section of thecylindrical storage battery. Inside the battery can 602 having a hollowcylindrical shape, a battery element in which a strip-like positiveelectrode 604 and a strip-like negative electrode 606 are wound with astripe-like separator 605 interposed therebetween is provided. Althoughnot illustrated, the battery element is wound around a center pin. Oneend of the battery can 602 is close and the other end thereof is open.For the battery can 602, a metal having a corrosion-resistant propertyto an electrolyte solution, such as nickel or titanium, an alloy of sucha metal, or an alloy of such a metal and another metal (e.g., stainlesssteel or the like) can be used. Alternatively, the battery can 602 ispreferably covered with nickel or the like in order to inhibit corrosiondue to the electrolyte solution. Inside the battery can 602, the batteryelement in which the positive electrode, the negative electrode, and theseparator are wound is provided between a pair of insulating plates 608and 609 which face each other. Furthermore, a nonaqueous electrolytesolution (not illustrated) is injected inside the battery can 602provided with the battery element. As the nonaqueous electrolytesolution, a nonaqueous electrolyte solution that is similar to that ofthe coin-type storage battery can be used.

Although the positive electrode 604 and the negative electrode 606 canbe formed in a manner similar to that of the positive electrode and thenegative electrode of the coin-type storage battery described above, thedifference lies in that, since the positive electrode and the negativeelectrode of the cylindrical storage battery are wound, active materialsare formed on both sides of the current collectors. At least one of thepositive electrode 604 and the negative electrode 606 is wrapped in agraphene oxide film (not illustrated). In the storage battery includingwound inner structures, the graphene oxide film can reduce frictiongenerated between the structures and relieve stress generatedtherebetween. A positive electrode terminal (positive electrode tabelectrode) 603 is connected to the positive electrode 604, and anegative electrode terminal (negative electrode tab) 607 is connected tothe negative electrode 606. Both the positive electrode terminal 603 andthe negative electrode terminal 607 can be formed using a metal materialsuch as aluminum. The positive electrode terminal 603 and the negativeelectrode terminal 607 are resistance-welded to a safety valve mechanism612 and the bottom of the battery can 602, respectively. The safetyvalve mechanism 612 is electrically connected to the positive electrodecap 601 through a positive temperature coefficient (PTC) element 611.The safety valve mechanism 612 cuts off electrical connection betweenthe positive electrode cap 601 and the positive electrode 604 when theinternal pressure of the battery exceeds a predetermined thresholdvalue. The PTC element 611, which serves as a thermally sensitiveresistor whose resistance increases as the temperature rises, limits theamount of current by increasing the resistance, in order to inhibitabnormal heat generation. Note that barium titanate (BaTiO₃)-basedsemiconductor ceramic can be used for the PTC element.

<<Laminated Storage Battery>>

Next, an example of a laminated storage battery will be described withreference to FIG. 11A. When a flexible laminated storage battery is usedin an electronic device at least part of which is flexible, the storagebattery can be bent as the electronic device is bent.

A laminated storage battery 500 illustrated in FIG. 11A includes apositive electrode 503 including a positive electrode current collector501 and a positive electrode active material layer 502, a negativeelectrode 506 including a negative electrode current collector 504 and anegative electrode active material layer 505, a separator 507, anelectrolyte solution 508, and an exterior body 509. The separator 507 isprovided between the positive electrode 503 and the negative electrode506 in the exterior body 509. The electrolyte solution 508 is includedin the exterior body 509. The electrolyte solution described inEmbodiment 1 can be used for the electrolyte solution 508. At least oneof the positive electrode 503 and the negative electrode 506 is wrappedin a graphene oxide film (not illustrated in FIGS. 11A and 11B).

In the laminated storage battery 500 illustrated in FIG. 11A, thepositive electrode current collector 501 and the negative electrodecurrent collector 504 also serve as terminals for an electrical contactwith an external portion. For this reason, each of the positiveelectrode current collector 501 and the negative electrode currentcollector 504 may be arranged so that part of the positive electrodecurrent collector 501 and part of the negative electrode currentcollector 504 are exposed to the outside of the exterior body 509.Alternatively, a tab electrode and the positive electrode currentcollector 501 or the negative electrode current collector 504 may bebonded to each other by ultrasonic welding, and instead of the positiveelectrode current collector 501 and the negative electrode currentcollector 504, the tab electrode may be exposed to the outside of theexterior body 509.

As the exterior body 509 in the laminated storage battery 500, forexample, a laminate film having a three-layer structure in which ahighly flexible metal thin film of aluminum, stainless steel, copper,nickel, or the like is provided over a film formed of a material such aspolyethylene, polypropylene, polycarbonate, ionomer, or polyamide, andan insulating synthetic resin film of a polyamide-based resin, apolyester-based resin, or the like is provided as the outer surface ofthe exterior body over the metal thin film can be used.

FIG. 11B illustrates an example of a cross-sectional structure of thelaminated storage battery 500. FIG. 11A illustrates an example ofincluding two electrodes for simplicity, and the actual battery includesa plurality of electrodes.

The example in FIG. 11B includes 16 electrodes. The laminated storagebattery 500 has flexibility even though including 16 electrodes. In FIG.11B, 8 negative electrodes 506 and 8 positive electrodes 503 areincluded. Note that FIG. 11B illustrates a cross section of the leadportion of the negative electrode, and 8 negative electrode currentcollectors 504 are bonded to each other by ultrasonic welding. It isneedless to say that the number of electrodes is not limited to 16, andmay be more than 16 or less than 16. In the case of using a large numberof electrodes, the storage battery can have high capacity. In contrast,in the case of using a small number of electrodes, the storage batterycan have a small thickness and high flexibility.

FIG. 12 and FIG. 13 each illustrate an example of the external view ofthe laminated storage battery 500. In FIG. 12 and FIG. 13 , the positiveelectrode 503, the negative electrode 506, the separator 507, theexterior body 509, a graphene oxide film 531, a positive electrode tabelectrode 510, and a negative electrode tab electrode 511 are included.Note that although a plurality of negative electrodes 506 and aplurality of positive electrodes 503 are included in the laminatedstorage battery, one negative electrode 506 and one positive electrode503 are shown in FIG. 12 and FIG. 13 for avoiding complexity.

FIG. 14A illustrates the external views of the positive electrode 503and the negative electrode 506. The positive electrode 503 includes thepositive electrode current collector 501, and the positive electrodeactive material layer 502 is formed on a surface of the positiveelectrode current collector 501. The positive electrode 503 alsoincludes an exposed region of the positive electrode current collector501. The region is a region connected to the tab electrode or a regionfunctioning as the tab electrode and is referred to as a tab region. Thenegative electrode 506 includes the negative electrode current collector504, and the negative electrode active material layer 505 is formed on asurface of the negative electrode current collector 504. The negativeelectrode 506 also includes an exposed region of the negative electrodecurrent collector 504, that is, a tab region. The areas and the shapesof the tab regions included in the positive electrode and the negativeelectrode are not limited to those illustrated in FIG. 14A. At least oneof the negative electrode 506 and the positive electrode 503 is wrappedin a graphene oxide film.

<<Method for Fabricating Laminated Storage Battery>>

Here, an example of a method for fabricating the laminated storagebattery whose external view is illustrated in FIG. 12 will be describedwith reference to FIGS. 14B and 14C.

First, the negative electrode 506, the separator 507, and the positiveelectrode 503 are stacked. FIG. 14B illustrates a stack including thenegative electrode 506, the separator 507, and the positive electrode503. The battery described here as an example includes 5 negativeelectrodes and 4 positive electrodes. Next, the tab regions of thepositive electrodes 503 are bonded to each other, and the tab region ofthe positive electrode of the outermost surface and the positiveelectrode tab electrode 510 are bonded to each other. The bonding can beperformed by ultrasonic welding, for example. In a similar manner, thetab regions of the negative electrodes 506 are bonded to each other, andthe tab region of the negative electrode of the outermost surface andthe negative electrode tab electrode 511 are bonded to each other. Inthis example, the negative electrode 506 is wrapped in the grapheneoxide film 531.

After that, the negative electrode 506, the separator 507, and thepositive electrode 503 are placed over the exterior body 509.

Subsequently, the exterior body 509 is folded along a dashed line asillustrated in FIG. 14C. Then, the outer edge of the exterior body 509is bonded. The bonding can be performed by thermocompression, forexample. At this time, a part (or one side) of the exterior body 509 isleft unbonded (to provide an inlet) so that the electrolyte solution 508can be introduced later.

Next, the electrolyte solution 508 is introduced into the exterior body509 from the inlet of the exterior body 509. The electrolyte solution508 is preferably introduced in a reduced pressure atmosphere or in aninert gas atmosphere. Lastly, the inlet is bonded. In the above manner,the laminated storage battery 500 can be fabricated.

Note that, in this embodiment, the coin-type storage battery, thelaminated storage battery, and the cylindrical storage battery are givenas examples of the storage battery; however, any of storage batterieswith a variety of shapes, such as a sealed storage battery and asquare-type storage battery, can be used. Furthermore, a structure inwhich a plurality of positive electrodes, a plurality of negativeelectrodes, and a plurality of separators are stacked or wound may beemployed.

FIGS. 15A to 15E illustrate examples of electronic devices includinglaminated storage batteries. Examples of electronic devices eachincluding a flexible storage battery include television devices (alsoreferred to as televisions or television receivers), monitors ofcomputers or the like, cameras such as digital cameras and digital videocameras, digital photo frames, mobile phones (also referred to ascellular phones or mobile phone devices), portable game machines,portable information terminals, audio reproducing devices, and largegame machines such as pachinko machines.

In addition, a flexible storage battery can be incorporated along acurved inside/outside wall surface of a house or a building or a curvedinterior/exterior surface of an automobile.

FIG. 15A illustrates an example of a mobile phone. A mobile phone 7400is provided with a display portion 7402 incorporated in a housing 7401,an operation button 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400includes a storage battery 7407.

FIG. 15B illustrates the mobile phone 7400 that is bent. When the wholemobile phone 7400 is bent by the external force, the storage battery7407 included in the mobile phone 7400 is also bent. FIG. 15Cillustrates the bent storage battery 7407. The storage battery 7407 is alaminated storage battery.

FIG. 15D illustrates an example of a bangle display device. A portabledisplay device 7100 includes a housing 7101, a display portion 7102, anoperation button 7103, and a storage battery 7104. FIG. 15E illustratesthe bent storage battery 7104.

<<Structural Examples of Storage Battery>>

Structural examples of a storage battery will be described withreference to FIGS. 16A and 16B, FIGS. 17A1, 17A2, 17B1, and 17B2, FIGS.18A and 18B, FIGS. 19A and 19B, and FIG. 20 . Note that in each of FIGS.16A and 16B, FIGS. 17A1, 17A2, 17B1, and 17B2, FIGS. 18A and 18B, FIGS.19A and 19B, and FIG. 20 , a positive electrode or a negative electrodeof the storage battery is wrapped in a graphene oxide film, though thegraphene oxide film is not illustrated.

FIGS. 16A and 16B are external views of a storage battery. The storagebattery includes a circuit board 900 and a storage battery 913. A label910 is attached to the storage battery 913. As shown in FIG. 16B, thestorage battery further includes a terminal 951, a terminal 952, anantenna 914, and an antenna 915.

The circuit board 900 includes terminals 911 and a circuit 912. Theterminals 911 are connected to the terminals 951 and 952, the antennas914 and 915, and the circuit 912. Note that a plurality of terminals 911serving as a control signal input terminal, a power supply terminal, andthe like may be provided.

The circuit 912 may be provided on the rear surface of the circuit board900. The shape of each of the antennas 914 and 915 is not limited to acoil shape and may be a linear shape or a plate shape. Furthermore, aplanar antenna, an aperture antenna, a traveling-wave antenna, an EHantenna, a magnetic-field antenna, a dielectric antenna, or the like maybe used. Alternatively, the antenna 914 or the antenna 915 may be aflat-plate conductor. The flat-plate conductor can serve as one ofconductors for electric field coupling. That is, the antenna 914 or theantenna 915 can serve as one of two conductors of a capacitor. Thus,electric power can be transmitted and received not only by anelectromagnetic field or a magnetic field but also by an electric field.

The line width of the antenna 914 is preferably larger than that of theantenna 915. This makes it possible to increase the amount of electricpower received by the antenna 914.

The storage battery includes a layer 916 between the storage battery 913and the antennas 914 and 915. The layer 916 may have a function ofinhibiting an adverse effect on an electromagnetic field by the storagebattery 913. As the layer 916, for example, a magnetic body can be used.

Note that the structure of the storage battery is not limited to thatshown in FIGS. 16A and 16B.

For example, as shown in FIGS. 17A1 and 17A2, two opposite surfaces ofthe storage battery 913 in FIGS. 16A and 16B may be provided withrespective antennas. FIG. 17A1 is an external view showing one side ofthe opposite surfaces, and FIG. 17A2 is an external view showing theother side of the opposite surfaces. For portions similar to those inFIGS. 16A and 16B, a description of the storage battery illustrated inFIGS. 16A and 16B can be referred to as appropriate.

As illustrated in FIG. 17A1, the antenna 914 is provided on one of theopposite surfaces of the storage battery 913 with the layer 916interposed therebetween, and as illustrated in FIG. 17A2, the antenna915 is provided on the other of the opposite surfaces of the storagebattery 913 with a layer 917 interposed therebetween. The layer 917 mayhave a function of inhibiting an adverse effect on an electromagneticfield by the storage battery 913, for example. As the layer 917, forexample, a magnetic body can be used.

With the above structure, both of the antennas 914 and 915 can beincreased in size.

Alternatively, as illustrated in FIGS. 17B1 and 17B2, two oppositesurfaces of the storage battery 913 in FIGS. 16A and 16B may be providedwith different types of antennas. FIG. 17B1 is an external view showingone side of the opposite surfaces, and FIG. 17B2 is an external viewshowing the other side of the opposite surfaces. For portions similar tothose in FIGS. 16A and 16B, a description of the storage batteryillustrated in FIGS. 16A and 16B can be referred to as appropriate.

As illustrated in FIG. 17B1, the antennas 914 and 915 are provided onone of the opposite surfaces of the storage battery 913 with the layer916 interposed therebetween, and as illustrated in FIG. 17B2, an antenna918 is provided on the other of the opposite surfaces of the storagebattery 913 with the layer 917 interposed therebetween. The antenna 918has a function of communicating data with an external device, forexample. An antenna with a shape that can be applied to the antennas 914and 915, for example, can be used as the antenna 918. As a system forcommunication using the antenna 918 between the storage battery andanother device, a response method that can be used between the storagebattery and another device, such as NFC, can be employed.

Alternatively, as illustrated in FIG. 18A, the storage battery 913 inFIGS. 16A and 16B may be provided with a display device 920. The displaydevice 920 is electrically connected to the terminal 911 via a terminal919. It is possible that the label 910 is not provided in a portionwhere the display device 920 is provided. For portions similar to thosein FIGS. 16A and 16B, a description of the storage battery illustratedin FIGS. 16A and 16B can be referred to as appropriate.

The display device 920 can display, for example, an image showingwhether charge is being carried out, an image showing the amount ofstored power, or the like. As the display device 920, electronic paper,a liquid crystal display device, an electroluminescent (EL) displaydevice, or the like can be used. For example, the use of electronicpaper can reduce power consumption of the display device 920.

Alternatively, as illustrated in FIG. 18B, the storage battery 913illustrated in FIGS. 16A and 16B may be provided with a sensor 921. Thesensor 921 is electrically connected to the terminal 911 via a terminal922. For portions similar to those in FIGS. 16A and 16B, a descriptionof the storage battery illustrated in FIGS. 16A and 16B can be referredto as appropriate.

The sensor 921 has a function of measuring, for example, displacement,position, speed, acceleration, angular velocity, rotational frequency,distance, light, liquid, magnetism, temperature, chemical substance,sound, time, hardness, electric field, electric current, voltage,electric power, radiation, flow rate, humidity, gradient, oscillation,odor, or infrared rays. With the sensor 921, for example, data on anenvironment (e.g., temperature) where the storage battery is placed canbe determined and stored in a memory inside the circuit 912.

Furthermore, structural examples of the storage battery 913 will bedescribed with reference to FIGS. 19A and 19B and FIG. 20 .

The storage battery 913 illustrated in FIG. 19A includes a wound body950 provided with the terminals 951 and 952 inside a housing 930. Thewound body 950 is soaked in an electrolyte solution inside the housing930. The terminal 952 is in contact with the housing 930. An insulatoror the like inhibits contact between the terminal 951 and the housing930. Note that in FIG. 19A, the housing 930 divided into two pieces isillustrated for convenience; however, in the actual structure, the woundbody 950 is covered with the housing 930 and the terminals 951 and 952extend to the outside of the housing 930. For the housing 930, a metalmaterial or a resin material can be used.

Note that as illustrated in FIG. 19B, the housing 930 in FIG. 19A may beformed using a plurality of materials. For example, in the storagebattery 913 in FIG. 19B, a housing 930 a and a housing 930 b are bondedto each other and the wound body 950 is provided in a region surroundedby the housing 930 a and the housing 930 b.

For the housing 930 a, an insulating material such as an organic resincan be used. In particular, when a material such as an organic resin isused for the side on which an antenna is formed, blocking of an electricfield by the storage battery 913 can be inhibited. When an electricfield is not significantly blocked by the housing 930 a, an antenna suchas the antennas 914 and 915 may be provided inside the housing 930 a.For the housing 930 b, a metal material can be used, for example.

FIG. 20 illustrates the structure of the wound body 950. The wound body950 includes a negative electrode 931, a positive electrode 932, andseparators 933. The wound body 950 is obtained by winding a sheet of astack in which the negative electrode 931 overlaps with the positiveelectrode 932 with the separator 933 provided therebetween. Note that aplurality of stacks of the negative electrode 931, the positiveelectrode 932, and the separator 933 may be stacked.

The negative electrode 931 is connected to the terminal 911 in FIGS. 16Aand 16B via one of the terminals 951 and 952. The positive electrode 932is connected to the terminal 911 in FIGS. 16A and 16B via the other ofthe terminals 951 and 952.

<<Examples of Electrical Devices: Vehicles>>

Next, examples where a storage battery is used in a vehicle will bedescribed. The use of storage batteries in vehicles enables productionof next-generation clean energy vehicles such as hybrid electricvehicles (HEV), electric vehicles (EV), and plug-in hybrid electricvehicles (PHEV).

FIGS. 21A and 21B each illustrate an example of a vehicle using oneembodiment of the present invention. An automobile 8100 illustrated inFIG. 21A is an electric vehicle that runs on the power of an electricmotor. Alternatively, the automobile 8100 is a hybrid electric vehiclecapable of driving appropriately using either the electric motor or theengine. One embodiment of the present invention can provide a vehiclewhich can be repeatedly charged and discharged. The automobile 8100includes the storage battery. The storage battery is used not only fordriving the electric motor, but also for supplying electric power to alight-emitting device such as a headlight 8101 or an interior light (notillustrated).

The storage battery can also supply electric power to a display deviceof a speedometer, a tachometer, or the like included in the automobile8100. Furthermore, the storage battery can supply electric power to asemiconductor device included in the automobile 8100, such as anavigation system.

FIG. 21B illustrates an automobile 8200 including the storage battery.The automobile 8200 can be charged when the storage battery is suppliedwith electric power through external charging equipment by a plug-insystem, a contactless power feeding system, or the like. In FIG. 21B,the storage battery included in the automobile 8200 is charged with theuse of a ground-based charging apparatus 8021 through a cable 8022. Incharging, a given method may be employed as a charging method, thestandard of a connector, or the like as appropriate. The chargingapparatus 8021 may be a charging station provided in a commerce facilityor a power source in a house. For example, with the use of a plug-intechnique, the storage battery included in the automobile 8200 can becharged by being supplied with electric power from outside. The chargingcan be performed by converting AC electric power into DC electric powerthrough a converter such as an AC-DC converter.

Furthermore, although not illustrated, the vehicle may include a powerreceiving device so that it can be charged by being supplied withelectric power from an above-ground power transmitting device in acontactless manner. In the case of the contactless power feeding system,by fitting a power transmitting device in a road or an exterior wall,charging can be performed not only when the electric vehicle is stoppedbut also when driven. In addition, the contactless power feeding systemmay be utilized to perform transmission and reception of electric powerbetween vehicles. Furthermore, a solar cell may be provided in theexterior of the automobile to charge the storage battery when theautomobile stops or moves. To supply electric power in such acontactless manner, an electromagnetic induction method or a magneticresonance method can be used.

According to one embodiment of the present invention, the storagebattery can have improved cycle life and reliability. Furthermore,according to one embodiment of the present invention, the storagebattery itself can be made more compact and lightweight as a result ofimproved characteristics of the storage battery. The compact andlightweight storage battery contributes to a reduction in the weight ofa vehicle, and thus increases the mileage. Furthermore, the storagebattery included in the vehicle can be used as a power source forsupplying electric power to products other than the vehicle. In such acase, the use of a commercial power source can be avoided at peak timeof electric power demand.

Embodiment 4

A battery management unit (BMU) which can be combined with a batterycell including any of the storage batteries described in Embodiments 1to 3 and a transistor suitable for a circuit included in the batterymanagement unit are described with reference to FIG. 22 , FIGS. 23A to23C, FIG. 24 , FIG. 25 , FIGS. 26A to 26C, FIG. 27 , and FIG. 28 . Inthis embodiment, a battery management unit of a storage batteryincluding battery cells that are connected in series is particularlydescribed.

When a plurality of battery cells connected in series are charged anddischarged repeatedly, each battery cell has different capacity (outputvoltage) from one another in accordance with the variation inperformance among the battery cells. A discharge capacity of all of thebattery cells connected in series depends on a battery cell with lowcapacity. The variation in capacities reduces the capacity of thebattery cells at the time of discharging. Charging based on a batterycell with low capacity may cause insufficient charging. Charging basedon a battery cell with high capacity may cause overcharge.

Thus, the battery management unit of the storage battery includingbattery cells connected in series has a function of reducing variationin capacities among the battery cells which causes insufficient chargingor overcharge. Although circuit structures for reducing variation incapacities among the battery cells include a resistive type, a capacitortype, and an inductor type, a circuit structure which can reducevariation in capacities among the battery cells using transistors with alow off-state current is explained here as an example.

A transistor including an oxide semiconductor in its channel formationregion (an OS transistor) is preferably used as the transistor with alow off-state current. When an OS transistor with a low off-statecurrent is used in the circuit of the battery management unit of thestorage battery, the amount of electric charge leaking from a batterycan be reduced, and reduction in capacity over time can be suppressed.

As the oxide semiconductor used in the channel formation region, anIn-M-Zn oxide (M is Ga, Sn, Y, Zr, La, Ce, or Nd) is used. In the casewhere the atomic ratio of the metal elements of a target for forming anoxide semiconductor film is In:M:Zn=x₁:y₁:z₁, x₁/y₁ is preferablygreater than or equal to ⅓ and less than or equal to 6, furtherpreferably greater than or equal to 1 and less than or equal to 6, andz₁/y₁ is preferably greater than or equal to ⅓ and less than or equal to6, further preferably greater than or equal to 1 and less than or equalto 6. Note that when z₁/y₁ is greater than or equal to 1 and less thanor equal to 6, a CAAC-OS film as the oxide semiconductor film is easilyformed.

Here, the details of the CAAC-OS film are described.

The CAAC-OS film is one of oxide semiconductor films having a pluralityof c-axis aligned crystal parts.

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OSfilm, which is obtained using a transmission electron microscope (TEM),a plurality of crystal parts can be observed. However, in thehigh-resolution TEM image, a boundary between crystal parts, that is, agrain boundary is not clearly observed. Thus, in the CAAC-OS film, areduction in electron mobility due to the grain boundary is less likelyto occur.

According to the high-resolution cross-sectional TEM image of theCAAC-OS film observed in a direction substantially parallel to a samplesurface, metal atoms are arranged in a layered manner in the crystalparts. Each metal atom layer has a morphology reflecting unevenness of asurface over which the CAAC-OS film is formed (hereinafter, a surfaceover which the CAAC-OS film is formed is referred to as a formationsurface) or of a top surface of the CAAC-OS film, and is arrangedparallel to the formation surface or the top surface of the CAAC-OSfilm.

On the other hand, according to the high-resolution planar TEM image ofthe CAAC-OS film observed in a direction substantially perpendicular tothe sample surface, metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included in apart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ not appear at around36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic arrangement ofthe oxide semiconductor film by depriving the oxide semiconductor filmof oxygen and causes a decrease in crystallinity. Further, a heavy metalsuch as iron or nickel, argon, carbon dioxide, or the like has a largeatomic radius (molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially highly purifiedintrinsic” state. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has few carrier generationsources, and thus can have a low carrier density. Thus, a transistorincluding the oxide semiconductor film rarely has negative thresholdvoltage (is rarely normally on). The highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has fewcarrier traps. Accordingly, the transistor including the oxidesemiconductor film has little variation in electrical characteristicsand high reliability. Electric charge trapped by the carrier traps inthe oxide semiconductor film takes a long time to be released, and mightbehave like fixed electric charge. Thus, the transistor which includesthe oxide semiconductor film having high impurity concentration and ahigh density of defect states has unstable electrical characteristics insome cases.

With the use of the CAAC-OS film in a transistor, variation in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

Since the OS transistor has a wider band gap than a transistor includingsilicon in its channel formation region (a Si transistor), dielectricbreakdown is unlikely to occur when a high voltage is applied. Althougha voltage of several hundreds of volts is generated when battery cellsare connected in series, the above-described OS transistor is suitablefor a circuit of a battery management unit which is used for suchbattery cells in the storage battery.

FIG. 22 is an example of a block diagram of the storage battery. Astorage battery BT00 illustrated in FIG. 22 includes a terminal pairBT01, a terminal pair BT02, a switching control circuit BT03, aswitching circuit BT04, a switching circuit BT05, a voltagetransformation control circuit BT06, a voltage transformer circuit BT07,and a battery portion BT08 including a plurality of battery cells BT09connected in series.

In the storage battery BT00 illustrated in FIG. 22 , a portion includingthe terminal pair BT01, the terminal pair BT02, the switching controlcircuit BT03, the switching circuit BT04, the switching circuit BT05,the voltage transformation control circuit BT06, and the voltagetransformer circuit BT07 can be referred to as a battery managementunit.

The switching control circuit BT03 controls operations of the switchingcircuits BT04 and BT05. Specifically, the switching control circuit BT03selects battery cells to be discharged (a discharge battery cell group)and battery cells to be charged (a charge battery cell group) inaccordance with voltage measured for every battery cell BT09.

Furthermore, the switching control circuit BT03 outputs a control signalS1 and a control signal S2 on the basis of the selected dischargebattery cell group and the selected charge battery cell group. Thecontrol signal S1 is output to the switching circuit BT04. The controlsignal S1 controls the switching circuit BT04 so that the terminal pairBT01 and the discharge battery cell group are connected. In addition,the control signal S2 is output to the switching circuit BT05. Thecontrol signal S2 controls the switching circuit BT05 so that theterminal pair BT02 and the charge battery cell group are connected.

The switching control circuit BT03 generates the control signal S1 andthe control signal S2 on the basis of connection relation of theswitching circuit BT04, the switching circuit BT05, and the voltagetransformer circuit BT07 so that terminals having the same polarity areconnected with each other in the terminal pair BT02 and the chargebattery cell group.

An operation of the switching control circuit BT03 is described indetail.

First, the switching control circuit BT03 measures the voltage of eachof the plurality of battery cells BT09. Then, the switching controlcircuit BT03 determines that the battery cell BT09 having a voltagehigher than a predetermined threshold value is a high-voltage batterycell (high-voltage cell) and that the battery cell BT09 having a voltagelower than the predetermined threshold value is a low-voltage batterycell (low-voltage cell), for example.

As a method to determine whether a battery cell is a high-voltage cellor a low-voltage cell, any of various methods can be employed. Forexample, the switching control circuit BT03 may determine whether eachbattery cell BT09 is a high-voltage cell or a low-voltage cell on thebasis of the voltage of a battery cell BT09 having a highest voltage ora lowest voltage among the plurality of battery cells BT09. In thiscase, the switching control circuit BT03 can determine whether eachbattery cell BT09 is a high-voltage cell or a low-voltage cell bydetermining whether or not a ratio of a voltage of each battery cellBT09 to the reference voltage is the predetermined value or more. Then,the switching control circuit BT03 determines a charge battery cellgroup and a discharge battery cell group on the basis of thedetermination result.

Note that high-voltage cells and low-voltage cells are mixed in variousstates in the plurality of battery cells BT09. For example, theswitching control circuit BT03 selects a portion having the largestnumber of high-voltage cells connected in series as the dischargebattery cell group of mixed high-voltage cells and low-voltage cells.Furthermore, the switching control circuit BT03 selects a portion havingthe largest number of low-voltage cells connected in series as thecharge battery cell group. In addition, the switching control circuitBT03 may preferentially select battery cells BT09 which are nearovercharged or overdischarged as the discharge battery cell group or thecharge battery cell group.

Here, operation examples of the switching control circuit BT03 in thisembodiment are described with reference to FIGS. 23A to 23C. FIGS. 23Ato 23C illustrate operation examples of the switching control circuitBT03. Note that FIGS. 23A to 23C each illustrate the case where fourbattery cells BT09 are connected in series as an example for convenienceof explanation.

FIG. 23A shows the case where the relation of voltages Va, Vb, Vc, andVd is Va=Vb=Vc>Vd where the voltages Va, Vb, Vc, and Vd are voltages ofa battery cell a, a battery cell b, a battery cell c, and a battery celld, respectively. That is, a series of three high-voltage cells a to cand one low-voltage cell d are connected in series. In that case, theswitching control circuit BT03 selects the series of three high-voltagecells a to c as the discharge battery cell group. In addition, theswitching control circuit BT03 selects the low-voltage cell d as thecharge battery cell group.

Next, FIG. 23B shows the case where the relation of the voltages isVc>Va=Vb>>Vd. That is, a series of two low-voltage cells a and b, onehigh-voltage cell c, and one low-voltage cell d which is close tooverdischarge are connected in series. In that case, the switchingcontrol circuit BT03 selects the high-voltage cell c as the dischargebattery cell group. Since the low-voltage cell d is close tooverdischarge, the switching control circuit BT03 preferentially selectsthe low-voltage cell d as the charge battery cell group instead of theseries of two low-voltage cells a and b.

Lastly, FIG. 23C shows the case where the relation of the voltages isVa>Vb=Vc=Vd. That is, one high-voltage cell a and a series of threelow-voltage cells b to d are connected in series. In that case, theswitching control circuit BT03 selects the high-voltage cell a as thedischarge battery cell group. In addition, the switching control circuitBT03 selects the series of three low-voltage cells b to d as the chargebattery cell group.

On the basis of the determination result shown in the examples of FIGS.23A to 23C, the switching control circuit BT03 outputs the controlsignal S1 and the control signal S2 to the switching circuit BT04 andthe switching circuit BT05, respectively. Information showing thedischarge battery cell group being the connection destination of theswitching circuit BT04 is set in the control signal S1. Informationshowing the charge battery cell group being a connection destination ofthe switching circuit BT05 is set in the control signal S2.

The above is the detailed description of the operation of the switchingcontrol circuit BT03.

The switching circuit BT04 sets the discharge battery cell groupselected by the switching control circuit BT03 as the connectiondestination of the terminal pair BT01 in response to the control signalS1 output from the switching control circuit BT03.

The terminal pair BT01 includes a pair of terminals A1 and A2. Theswitching circuit BT04 sets the connection destination of the terminalpair BT01 by connecting one of the pair of terminals A1 and A2 to apositive electrode terminal of a battery cell BT09 positioned on themost upstream side (on the high potential side) of the discharge batterycell group, and the other to a negative electrode terminal of a batterycell BT09 positioned on the most downstream side (on the low potentialside) of the discharge battery cell group. Note that the switchingcircuit BT04 can recognize the position of the discharge battery cellgroup on the basis of the information set in the control signal S1.

The switching circuit BT05 sets the charge battery cell group selectedby the switching control circuit BT03 as the connection destination ofthe terminal pair BT02 in response to the control signal S2 output fromthe switching control circuit BT03.

The terminal pair BT02 includes a pair of terminals B1 and B2. Theswitching circuit BT05 sets the connection destination of the terminalpair BT02 by connecting one of the pair of terminals B1 and B2 to apositive electrode terminal of a battery cell BT09 positioned on themost upstream side (on the high potential side) of the charge batterycell group, and the other to a negative electrode terminal of a batterycell BT09 positioned on the most downstream side (on the low potentialside) of the charge battery cell group. Note that the switching circuitBT05 can recognize the position of the charge battery cell group on thebasis of the information set in the control signal S2.

FIG. 24 and FIG. 25 are circuit diagrams showing configuration examplesof the switching circuits BT04 and BT05.

In FIG. 24 , the switching circuit BT04 includes a plurality oftransistors BT10, a bus BT11, and a bus BT12. The bus BT11 is connectedto the terminal A1. The bus BT12 is connected to the terminal A2.Sources or drains of the plurality of transistors BT10 are connectedalternately to the bus BT11 and the bus BT12. Sources or drains whichare not connected to the bus BT11 and the bus BT12 of the plurality oftransistors BT10 are each connected between two adjacent battery cellsBT09.

A source or a drain of a transistor BT10 which is not connected to thebus BT11 on the most upstream side of the plurality of transistors BT10is connected to a positive electrode terminal of a battery cell BT09 onthe most upstream side of the battery portion BT08. A source or a drainof a transistor BT10 which is not connected to the bus BT11 on the mostdownstream side of the plurality of transistors BT10 is connected to anegative electrode terminal of a battery cell BT09 on the mostdownstream side of the battery portion BT08.

The switching circuit BT04 connects the discharge battery cell group tothe terminal pair BT01 by bringing one of the plurality of thetransistors BT10 which are connected to the bus BT11 and one of theplurality of transistors BT10 which are connected to the bus BT12 intoan on state in response to the control signal S1 supplied to gates ofthe plurality of transistors BT10. Accordingly, the positive electrodeterminal of the battery cell BT09 on the most upstream side of thedischarge battery cell group is connected to one of the pair ofterminals A1 and A2. In addition, the negative electrode terminal of thebattery cell BT09 on the most downstream side of the discharge batterycell group is connected to the other of the pair of terminals A1 and A2(i.e., a terminal which is not connected to the positive electrodeterminal).

An OS transistor is preferably used as the transistor BT10. Since theoff-state current of the OS transistor is low, the amount of electriccharge leaking from battery cells which do not belong to the dischargebattery cell group can be reduced, and reduction in capacity over timecan be suppressed. In addition, dielectric breakdown is unlikely tooccur in the OS transistor when a high voltage is applied. Therefore,the battery cell BT09 and the terminal pair BT01, which are connected tothe transistor BT10 in an off state, can be insulated from each othereven when an output voltage of the discharge battery cell group is high.

In FIG. 24 , the switching circuit BT05 includes a plurality oftransistors BT13, a current control switch BT14, a bus BT15, and a busBT16. The bus BT15 and the bus BT16 are provided between the pluralityof transistors BT13 and the current control switch BT14. Sources ordrains of the plurality of transistors BT13 are connected alternately tothe bus BT15 and the bus BT16. Sources or drains which are not connectedto the bus BT15 and the bus BT16 of the plurality of transistors BT13are each connected between two adjacent battery cells BT09.

A source or a drain of a transistor BT13 on the most upstream side ofthe plurality of transistors BT13 is connected to a positive electrodeterminal of the battery cell BT09 on the most upstream side of thebattery portion BT08. A source or a drain of a transistor BT13 on themost downstream side of the plurality of transistors BT13 is connectedto a negative electrode terminal of the battery cell BT09 on the mostdownstream side of the battery portion BT08.

OS transistors are preferably used as the transistors BT13 like thetransistors BT10. Since the off-state current of the OS transistor islow, the amount of electric charge leaking from the battery cells whichdo not belong to the charge battery cell group can be reduced, andreduction in capacity over time can be suppressed. In addition,dielectric breakdown is unlikely to occur in the OS transistor when ahigh voltage is applied. Therefore, the battery cell BT09 and theterminal pair BT02, which are connected to the transistor BT13 in an offstate, can be insulated from each other even when a voltage for chargingthe charge battery cell group is high.

The current control switch BT14 includes a switch pair BT17 and a switchpair BT18. One end of the switch pair BT17 is connected to the terminalB1. The other ends of the switch pair BT17 extend from respectiveswitches of the switch pair BT17. One switch is connected to the busBT15, and the other switch is connected to the bus BT16. One end of theswitch pair BT18 is connected to the terminal B2. The other ends of theswitch pair BT18 extend from two switches of the switch pair BT18. Oneswitch is connected to the bus BT15, and the other switch is connectedto the bus BT16.

OS transistors are preferably used for the switches included in theswitch pair BT17 and the switch pair BT18 like the transistors BT10 andBT13.

The switching circuit BT05 connects the charge battery cell group andthe terminal pair BT02 by controlling the combination of on and offstates of the transistors BT13 and the current control switch BT14 inresponse to the control signal S2.

For example, the switching circuit BT05 connects the charge battery cellgroup and the terminal pair BT02 in the following manner.

The switching circuit BT05 brings a transistor BT13 connected to apositive electrode terminal of a battery cell BT09 on the most upstreamside of the charge battery cell group into an on state in response tothe control signal S2 supplied to gates of the plurality of thetransistors BT13. In addition, the switching circuit BT05 brings atransistor BT13 connected to a negative electrode terminal of a batterycell BT09 on the most downstream side of the charge battery cell groupinto an on state in response to the control signal S2 supplied to thegates of the plurality of the transistors BT13.

The polarities of voltages applied to the terminal pair BT02 can vary inaccordance with the connection structures of the voltage transformercircuit BT07 and the discharge battery cell group connected to theterminal pair BT01. In order to supply current in a direction forcharging the charge battery cell group, terminals with the same polarityof the terminal pair BT02 and the charge battery cell group are requiredto be connected. In view of this, the current control switch BT14 iscontrolled by the control signal S2 so that the connection destinationof the switch pair BT17 and that of the switch pair BT18 are changed inaccordance with the polarities of the voltages applied to the terminalpair BT02.

The state where voltages are applied to the terminal pair BT02 so as tomake the terminal B1 a positive electrode and the terminal B2 a negativeelectrode is described as an example. Here, in the case where thebattery cell BT09 positioned on the most downstream side of the batteryportion BT08 is in the charge battery cell group, the switch pair BT17is controlled to be connected to the positive electrode terminal of thebattery cell BT09 in response to the control signal S2. That is, theswitch of the switch pair BT17 connected to the bus BT16 is turned on,and the switch of the switch pair BT17 connected to the bus BT15 isturned off. In contrast, the switch pair BT18 is controlled to beconnected to the negative electrode terminal of the battery cell BT09positioned on the most downstream side of the battery portion BT08 inresponse to the control signal S2. That is, the switch of the switchpair BT18 connected to the bus BT15 is turned on, and the switch of theswitch pair BT18 connected to the bus BT16 is turned off. In thismanner, terminals with the same polarity of the terminal pair BT02 andthe charge battery cell group are connected to each other. In addition,the current which flows from the terminal pair BT02 is controlled to besupplied in a direction so as to charge the charge battery cell group.

In addition, instead of the switching circuit BT05, the switchingcircuit BT04 may include the current control switch BT14.

FIG. 25 is a circuit diagram illustrating structure examples of theswitching circuit BT04 and the switching circuit BT05 which aredifferent from those of FIG. 24 .

In FIG. 25 , the switching circuit BT04 includes a plurality oftransistor pairs BT21, a bus BT24, and a bus BT25. The bus BT24 isconnected to the terminal A1. The bus BT25 is connected to the terminalA2. One end of each of the plurality of transistor pairs BT21 extendsfrom a transistor BT22 and a transistor BT23. A source or drain of thetransistor BT22 is connected to the bus BT24. A source or drain of thetransistor BT23 is connected to the bus BT25. In addition, the other endof each of the transistor pairs is connected between two adjacentbattery cells BT09. The other end of the transistor pair BT21 on themost upstream side of the plurality of transistor pairs BT21 isconnected to the positive electrode terminal of the battery cell BT09 onthe most upstream side of the battery portion BT08. The other end of thetransistor pair BT21 on the most downstream side of the plurality oftransistor pairs BT21 is connected to the negative electrode terminal ofthe battery cell BT09 on the most downstream side of the battery portionBT08.

The switching circuit BT04 switches the connection destination of thetransistor pair BT21 to one of the terminal A1 and the terminal A2 byturning on or off the transistors BT22 and BT23 in response to thecontrol signal S1. Specifically, when the transistor BT22 is turned on,the transistor BT23 is turned off, so that the connection destination ofthe transistor pair BT21 is the terminal A1. On the other hand, when thetransistor BT23 is turned on, the transistor BT22 is turned off, so thatthe connection destination of the transistor pair BT21 is the terminalA2. Which of the transistors BT22 and BT23 is turned on is determined bythe control signal Si.

Two transistor pairs BT21 are used to connect the terminal pair BT01 andthe discharge battery cell group. Specifically, the connectiondestinations of the two transistor pairs BT21 are determined on thebasis of the control signal S1, and the discharge battery cell group andthe terminal pair BT01 are connected. The connection destinations of thetwo transistor pairs BT21 are controlled by the control signal S1 sothat one of the connection destinations is the terminal A1 and the otheris the terminal A2.

The switching circuit BT05 includes a plurality of transistor pairsBT31, a bus BT34, and a bus BT35. The bus BT34 is connected to theterminal B1. The bus BT35 is connected to the terminal B2. One ends ofthe transistor pairs BT31 extend from a transistor BT32 and a transistorBT33. One end extending from the transistor BT32 is connected to the busBT34. One end extending from the transistor BT33 is connected to the busBT35. The other end of each of the transistor pairs BT31 is connectedbetween two adjacent battery cells BT09. The other end of the transistorpair BT31 on the most upstream side of the plurality of transistor pairsBT31 is connected to the positive electrode terminal of the battery cellBT09 on the most upstream side of the battery portion BT08. The otherend of the transistor pair BT31 on the most downstream side of theplurality of transistor pairs BT31 is connected to the negativeelectrode terminal of the battery cell BT09 on the most downstream sideof the battery portion BT08.

The switching circuit BT05 switches the connection destination of thetransistor pair BT31 to one of the terminal B1 and the terminal B2 byturning on or off the transistors BT32 and BT33 in response to thecontrol signal S2. Specifically, when the transistor BT32 is turned on,the transistor BT33 is turned off, so that the connection destination ofthe transistor pair BT31 is the terminal B1. On the other hand, when thetransistor BT33 is turned on, the transistor BT32 is turned off, so thatthe connection destination of the transistor pair BT31 is the terminalB2. Which of the transistors BT32 and BT33 is turned on is determined bythe control signal S2.

Two transistor pairs BT31 are used to connect the terminal pair BT02 andthe charge battery cell group. Specifically, the connection destinationsof the two transistor pairs BT31 are determined on the basis of thecontrol signal S2, and the charge battery cell group and the terminalpair BT02 are connected. The connection destinations of the twotransistor pairs BT31 are controlled by the control signal S2 so thatone of the connection destinations is the terminal B1 and the other isthe terminal B2.

The connection destinations of the two transistor pairs BT31 aredetermined by the polarities of the voltages applied to the terminalpair BT02. Specifically, in the case where voltages which make theterminal B1 a positive electrode and the terminal B2 a negativeelectrode are applied to the terminal pair BT02, the transistor pairBT31 on the upstream side is controlled by the control signal S2 so thatthe transistor BT32 is turned on and the transistor BT33 is turned off.In contrast, the transistor pair BT31 on the downstream side iscontrolled by the control signal S2 so that the transistor BT33 isturned on and the transistor BT32 is turned off. In the case wherevoltages which make the terminal B1 a negative electrode and theterminal B2 a positive electrode are applied to the terminal pair BT02,the transistor pair BT31 on the upstream side is controlled by thecontrol signal S2 so that the transistor BT33 is turned on and thetransistor BT32 is turned off. In contrast, the transistor pair BT31 onthe downstream side is controlled by the control signal S2 so that thetransistor BT32 is turned on and the transistor BT33 is turned off. Inthis manner, terminals with the same polarity of the terminal pair BT02and the charge battery cell group are connected to each other. Inaddition, the current which flows from the terminal pair BT02 iscontrolled to be supplied in a direction for charging the charge batterycell group.

The voltage transformation control circuit BT06 controls operation ofthe voltage transformer circuit BT07. The voltage transformation controlcircuit BT06 generates a voltage transformation signal S3 forcontrolling the operation of the voltage transformer circuit BT07 on thebasis of the number of the battery cells BT09 included in the dischargebattery cell group and the number of the battery cells BT09 included inthe charge battery cell group and outputs the voltage transformationsignal S3 to the voltage transformer circuit BT07.

In the case where the number of the battery cells BT09 included in thedischarge battery cell group is larger than that included in the chargebattery cell group, it is necessary to inhibit a charging voltage whichis too high from being applied to the charge battery cell group. Thus,the voltage transformation control circuit BT06 outputs the voltagetransformation signal S3 for controlling the voltage transformer circuitBT07 so that a discharging voltage (Vdis) is lowered within a rangewhere the charge battery cell group can be charged.

In the case where the number of the battery cells BT09 included in thedischarge battery cell group is less than or equal to that included inthe charge battery cell group, a voltage necessary for charging thecharge battery cell group needs to be secured. Therefore, the voltagetransformation control circuit BT06 outputs the voltage transformationsignal S3 for controlling the voltage transformer circuit BT07 so thatthe discharging voltage (Vdis) is raised within a range where a chargingvoltage which is too high is not applied to the charge battery cellgroup.

The voltage value of the charging voltage which is too high isdetermined in the light of product specifications and the like of thebattery cell BT09 used in the battery portion BT08. The voltage which israised or lowered by the voltage transformer circuit BT07 is applied asa charging voltage (Vcha) to the terminal pair BT02.

Here, operation examples of the voltage transformation control circuitBT06 in this embodiment are described with reference to FIGS. 26A to26C. FIGS. 26A to 26C are conceptual diagrams for explaining theoperation examples of the voltage transformation control circuits BT06for the discharge battery cell groups and the charge battery cell groupsdescribed in FIGS. 23A to 23C. FIGS. 26A to 26C each illustrate abattery management unit BT41. The battery management unit BT41 includesthe terminal pair BT01, the terminal pair BT02, the switching controlcircuit BT03, the switching circuit BT04, the switching circuit BT05,the voltage transformation control circuit BT06, and the voltagetransformer circuit BT07.

In an example illustrated in FIG. 26A, the series of three high-voltagecells a to c and one low-voltage cell d are connected in series asdescribed in FIG. 23A. In that case, as described using FIG. 23A, theswitching control circuit BT03 selects the high-voltage cells a to c asthe discharge battery cell group, and selects the low-voltage cell d asthe charge battery cell group. The voltage transformation controlcircuit BT06 calculates a conversion ratio N for converting thedischarging voltage (Vdis) to the charging voltage (Vcha) on the basisof the ratio of the number of the battery cells BT09 included in thecharge battery cell group to the number of the battery cells BT09included in the discharge battery cell group.

In the case where the number of the battery cells BT09 included in thedischarge battery cell group is larger than that included in the chargebattery cell group, when a discharging voltage is applied to theterminal pair BT02 without transforming the voltage, overvoltage may beapplied to the battery cells BT09 included in the charge battery cellgroup through the terminal pair BT02. Thus, in the case of FIG. 26A, itis necessary that a charging voltage (Vcha) applied to the terminal pairBT02 be lowered than the discharging voltage. In addition, in order tocharge the charge battery cell group, it is necessary that the chargingvoltage be higher than the total voltage of the battery cells BT09included in the charge battery cell group. Thus, the transformationcontrol circuit BT06 sets the conversion ratio N larger than the ratioof the number of the battery cells BT09 included in the charge batterycell group to the number of the battery cells BT09 included in thedischarge battery cell group.

Thus, the voltage transformation control circuit BT06 preferably setsthe conversion ratio N larger than the ratio of the number of thebattery cells BT09 included in the charge battery cell group to thenumber of the battery cells BT09 included in the discharge battery cellgroup by about 1% to 10%. Here, the charging voltage is made higher thanthe voltage of the charge battery cell group, but the charging voltageis equal to the voltage of the charge battery cell group in reality.Note that the voltage transformation control circuit BT06 feeds acurrent for charging the charge battery cell group in accordance withthe conversion ratio N in order to make the voltage of the chargebattery cell group equal to the charging voltage. The value of thecurrent is set by the voltage transformation control circuit BT06.

In the example illustrated in FIG. 26A, since the number of the batterycells BT09 included in the discharge battery cell group is three and thenumber of the battery cells BT09 included in the charge battery cellgroup is one, the voltage transformation control circuit BT06 calculatesa value which is slightly larger than ⅓ as the conversion ratio N. Then,the voltage transformation control circuit BT06 outputs the voltagetransformation signal S3, which lowers the discharging voltage inaccordance with the conversion ratio N and converts the voltage into acharging voltage, to the voltage transformer circuit BT07. The voltagetransformer circuit BT07 applies the charging voltage which istransformed in response to the voltage transformation signal S3 to theterminal pair BT02. Then, the battery cells BT09 included in the chargebattery cell group are charged with the charging voltage applied to theterminal pair BT02.

In each of examples illustrated in FIGS. 26B and 26C, the conversionratio N is calculated in a manner similar to that of FIG. 26A. In eachof the examples illustrated in FIGS. 26B and 26C, since the number ofthe battery cells BT09 included in the discharge battery cell group isless than or equal to the number of the battery cells BT09 included inthe charge battery cell group, the conversion ratio N is 1 or more.Therefore, in this case, the voltage transformation control circuit BT06outputs the voltage transformation signal S3 for raising the dischargingvoltage and converting the voltage into the charging voltage.

The voltage transformer circuit BT07 converts the discharging voltageapplied to the terminal pair BT01 into a charging voltage on the basisof the voltage transformation signal S3. The voltage transformer circuitBT07 applies the converted charging voltage to the terminal pair BT02.Here, the voltage transformer circuit BT07 electrically insulates theterminal pair BT01 from the terminal pair BT02. Accordingly, the voltagetransformer circuit BT07 inhibits a short-circuit due to a differencebetween the absolute voltage of the negative electrode terminal of thebattery cell BT09 on the most downstream side of the discharge batterycell group and the absolute voltage of the negative electrode terminalof the battery cell BT09 on the most downstream side of the chargebattery cell group. Furthermore, the voltage transformer circuit BT07converts the discharging voltage, which is the total voltage of thedischarge battery cell group, into the charging voltage on the basis ofthe voltage transformation signal S3 as described above.

An insulated direct current (DC)-DC converter or the like can be used inthe voltage transformer circuit BT07. In that case, the voltagetransformation control circuit BT06 controls the charging voltageconverted by the voltage transformer circuit BT07 by outputting a signalfor controlling the on/off ratio (the duty ratio) of the insulated DC-DCconverter as the voltage transformation signal S3.

Examples of the insulated DC-DC converter include a flyback converter, aforward converter, a ringing choke converter (RCC), a push-pullconverter, a half-bridge converter, and a full-bridge converter, and asuitable converter is selected in accordance with the value of theintended output voltage.

The structure of the voltage transformer circuit BT07 including theinsulated DC-DC converter is illustrated in FIG. 27 . An insulated DC-DCconverter BT51 includes a switch portion BT52 and a transformer BT53.The switch portion BT52 is a switch for switching on/off of theoperation of the insulated DC-DC converter, and a metal oxidesemiconductor field-effect transistor (MOSFET), a bipolar transistor, orthe like is used as the switch portion BT52. The switch portion BT52periodically turns on and off the insulated DC-DC converter BT51 inaccordance with the voltage transformation signal S3 controlling theon/off ratio which is output from the voltage transformation controlcircuit BT06. The switch portion BT52 can have any of various structuresin accordance with the type of the insulated DC-DC converter which isused. The transformer BT53 converts the discharging voltage applied fromthe terminal pair BT01 into the charging voltage. In detail, thetransformer BT53 operates in conjunction with the on/off state of theswitch portion BT52 and converts the discharging voltage into thecharging voltage in accordance with the on/off ratio. As the time duringwhich the switch portion BT52 is on becomes longer in its switchingperiod, the charging voltage is increased. On the other hand, as thetime during which the switch portion BT52 is on becomes shorter in itsswitching period, the charging voltage is decreased. In the case wherethe insulated DC-DC converter is used, the terminal pair BT01 and theterminal pair BT02 can be insulated from each other inside thetransformer BT53.

A flow of operation of the storage battery BT00 in this embodiment isdescribed with reference to FIG. 28 . FIG. 28 is a flow chartillustrating the flow of the operation of the storage battery BT00.

First, the storage battery BT00 obtains a voltage measured for each ofthe plurality of battery cells BT09 (step S001). Then, the storagebattery BT00 determines whether or not the condition for starting theoperation of reducing variation in voltages of the plurality of thebattery cells BT09 is satisfied (step S002). An example of the startingcondition can be that the difference between the maximum value and theminimum value of the voltage measured for each of the plurality of thebattery cells BT09 is higher than or equal to the predeterminedthreshold value. In the case where the starting condition is notsatisfied (step S002: NO), the storage battery BT00 does not perform thefollowing operation because voltages of the battery cells BT09 are wellbalanced. In contrast, in the case where the condition is satisfied(step S002: YES), the storage battery BT00 performs the operation ofreducing variation in the voltages of the battery cells BT09. In thisoperation, the storage battery BT00 determines whether each battery cellBT09 is a high-voltage cell or a low-voltage cell on the basis of themeasured voltage of each cell (step S003). Then, the storage batteryBT00 determines a discharge battery cell group and a charge battery cellgroup on the basis of the determination result (step S004). In addition,the storage battery BT00 generates the control signal S1 for setting thedetermined discharge battery cell group as the connection destination ofthe terminal pair BT01, and the control signal S2 for setting thedetermined charge battery cell group as the connection destination ofthe terminal pair BT02 (step S005). The storage battery BT00 outputs thegenerated control signals Si and S2 to the switching circuit BT04 andthe switching circuit BT05, respectively. Then, the switching circuitBT04 connects the terminal pair BT01 and the discharge battery cellgroup, and the switching circuit BT05 connects the terminal pair BT02and the discharge battery cell group (step S006). The storage batteryBT00 generates the voltage transformation signal S3 based on the numberof the battery cells BT09 included in the discharge battery cell groupand the number of the battery cells BT09 included in the charge batterycell group (step S007). Then, the storage battery BT00 converts thedischarging voltage applied to the terminal pair BT01 into a chargingvoltage based on the voltage transformation signal S3 and applies thecharging voltage to the terminal pair BT02 (step S008). In this way,electric charge of the discharge battery cell group is transferred tothe charge battery cell group.

Although the plurality of steps are shown in order in the flow chart ofFIG. 28 , the order of performing the steps is not limited to the order.

According to the above embodiment, when an electric charge istransferred from the discharge battery cell group to the charge batterycell group, a structure where an electric charge from the dischargebattery cell group is temporarily stored, and the stored electric chargeis sent to the charge battery cell group is unnecessary, unlike in the acapacitor type circuit. Accordingly, the charge transfer efficiency perunit time can be increased. In addition, the switching circuit BT04 andthe switching circuit BT05 determine which battery cell in the dischargebattery cell group and the charge battery cell group to be connected tothe voltage transformer circuit.

Furthermore, the voltage transformer circuit BT07 converts thedischarging voltage applied to the terminal pair BT01 into the chargingvoltage based on the number of the battery cells BT09 included in thedischarge battery cell group and the number of the battery cells BT09included in the charge battery cell group, and applies the chargingvoltage to the terminal pair BT02. Thus, even when any battery cell BT09is selected as the discharge battery cell group and the charge batterycell group, an electric charge can be transferred without any problems.

Furthermore, the use of OS transistors as the transistor BT10 and thetransistor BT13 can reduce the amount of electric charge leaking fromthe battery cells BT09 which do not belong to the charge battery cellgroup or the discharge battery cell group. Accordingly, a decrease incapacity of the battery cells BT09 which do not contribute to chargingor discharging can be suppressed. In addition, the variation incharacteristics of the OS transistor due to heat is smaller than that ofan Si transistor. Accordingly, even when the temperature of the batterycells BT09 is increased, an operation such as turning on or off thetransistors in response to the control signals S1 and S2 can beperformed normally.

Note that this embodiment can be implemented by being combined withother embodiments as appropriate.

This application is based on Japanese Patent Application serial no.2015-134671 filed with Japan Patent Office on Jul. 3, 2015, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. An electronic device comprising: a housing; a displayportion incorporated in the housing; and a flexible storage batteryincorporated in the housing, wherein the flexible storage batterycomprises: a positive electrode; a negative electrode; a folded sheetcomprising a graphene compound; and an exterior body, wherein at least aregion of the positive electrode is sandwiched between the folded sheet,and wherein the positive electrode and the negative electrode are storedin the exterior body.
 3. An electronic device comprising: a housing; adisplay portion incorporated in the housing; and a flexible storagebattery incorporated in the housing, wherein the flexible storagebattery comprises: a positive electrode; a negative electrode; a foldedsheet comprising a graphene compound; and an exterior body, wherein atleast a region of the negative electrode is sandwiched between thefolded sheet, and wherein the positive electrode and the negativeelectrode are stored in the exterior body.
 4. An electronic devicecomprising: a housing; a display portion incorporated in the housing;and a flexible storage battery incorporated in the housing, wherein theflexible storage battery comprises: a positive electrode; a negativeelectrode; a first folded sheet comprising a graphene compound; a secondfolded sheet comprising a graphene compound; and an exterior body,wherein at least a region of the positive electrode is sandwichedbetween the first folded sheet, wherein at least a region of thenegative electrode is sandwiched between the second folded sheet, andwherein the positive electrode and the negative electrode are stored inthe exterior body.
 5. The electronic device according to claim 2,wherein the positive electrode comprises a positive electrode currentcollector and a positive electrode active material layer, wherein thenegative electrode comprises a negative electrode current collector anda negative electrode active material layer, and wherein a material ofthe positive electrode current collector and a material of the positiveelectrode active material layer are different.
 6. The electronic deviceaccording to claim 3, wherein the positive electrode comprises apositive electrode current collector and a positive electrode activematerial layer, wherein the negative electrode comprises a negativeelectrode current collector and a negative electrode active materiallayer, and wherein a material of the positive electrode currentcollector and a material of the positive electrode active material layerare different.
 7. The electronic device according to claim 4, whereinthe positive electrode comprises a positive electrode current collectorand a positive electrode active material layer, wherein the negativeelectrode comprises a negative electrode current collector and anegative electrode active material layer, and wherein a material of thepositive electrode current collector and a material of the positiveelectrode active material layer are different.
 8. The electronic deviceaccording to claim 2, wherein the flexible storage battery is a flexiblelaminated storage battery.
 9. The electronic device according to claim3, wherein the flexible storage battery is a flexible laminated storagebattery.
 10. The electronic device according to claim 4, wherein theflexible storage battery is a flexible laminated storage battery. 11.The electronic device according to claim 2, further comprising aseparator between the positive electrode and the negative electrode, theseparator being stored in the exterior body.
 12. The electronic deviceaccording to claim 3, further comprising a separator between thepositive electrode and the negative electrode, the separator beingstored in the exterior body.
 13. The electronic device according toclaim 4, further comprising a separator between the positive electrodeand the negative electrode, the separator being stored in the exteriorbody.
 14. The electronic device according to claim 2, wherein thegraphene compound comprises graphene and oxygen.
 15. The electronicdevice according to claim 3, wherein the graphene compound comprisesgraphene and oxygen.
 16. The electronic device according to claim 4,wherein the graphene compound comprises graphene and oxygen.
 17. Theelectronic device according to claim 2, wherein the flexible storagebattery is bent in the electronic device which is bent.
 18. Theelectronic device according to claim 3, wherein the flexible storagebattery is bent in the electronic device which is bent.
 19. Theelectronic device according to claim 4, wherein the flexible storagebattery is bent in the electronic device which is bent.